Infection is a condition where harmful microorganisms invade the body, multiply, and trigger a reaction from the body’s tissues. These tiny living beings can be bacteria, viruses, fungi, or parasites that enter through various routes and cause illness ranging from mild discomfort to life-threatening conditions.

Understanding Infection

An infection happens when microscopic organisms known as pathogens enter the body, increase in number, and cause the body to react. These pathogens are too small to see with the naked eye, but they can have powerful effects on human health. While millions of microorganisms live around us and even on our bodies without causing harm, only a small portion of germs can actually cause infection.^[1][16]

Not all microorganisms are dangerous. In fact, many organisms that naturally live on the skin, in the nose and throat, and in the digestive tract are harmless or even helpful. These are called normal flora. Every person carries their own individual collection of microorganisms in and on their body, referred to as their microbiome. This personal microbiome is acquired at birth and changes throughout life. It differs from one body part to another and from one person to another. Scientists have recently discovered that a person’s gut microbiome can actually impact their immune system.^[7]

However, under certain conditions, some organisms can cause disease. When harmful pathogens enter the body through openings like the nose, mouth, ears, or through cuts in the skin, they can multiply quickly. The symptoms people experience during an infection often occur because the immune system is responding to the pathogen and trying to fight it off.^[4]

Epidemiology

Infectious diseases remain a significant global health concern affecting millions of people worldwide. In 2013, infections caused approximately 9.2 million deaths globally, representing 17 percent of all deaths that year.^[1] This substantial number highlights that despite advances in medicine and public health, infectious diseases continue to be a major cause of illness and death around the world.

Almost 1.7 million people in the United States develop sepsis each year, which is a life-threatening condition that happens when the body’s response to an infection spirals out of control. Even with modern treatments, sepsis kills nearly 270,000 of those affected.^[18] This demonstrates that severe infections can have devastating consequences even in countries with advanced healthcare systems.

Infections are common occurrences in everyday life. Everyone gets infectious diseases at some point, from respiratory infections like colds, flu, and COVID-19, to stomach flu, sexually transmitted infections, and even common foot fungus.^[2] The prevalence of infections affects people of all ages and backgrounds, making prevention and proper treatment important public health priorities.

Infections can be acquired at home or in the community and brought into various settings, or they can be acquired and spread within healthcare facilities, schools, or workplaces. These infections are especially common in children and young people’s settings, where germs can spread easily among groups of people in close contact.^[9]

Causes

Infections are caused by infectious agents called pathogens that include bacteria, viruses, fungi, parasites, and rarely, prions. Each type of pathogen has different characteristics in terms of size, shape, function, genetic makeup, how it acts on the body, and how it should be treated.^[1][2]

Viruses are among the most common causes of infection in humans. Researchers estimate there may be 10 nonillion viruses on Earth, with around 270 of those affecting humans. A nonillion is a 1 followed by 30 zeros. Viruses are made up of a small piece of genetic code, either DNA or RNA, protected by a coat of protein and fat molecules. After a virus enters a host, it invades the body’s cells and uses the components of those cells to replicate and produce more viruses. When the replication cycle is complete, new viruses are released into the body, causing damage or destruction of the host’s cells.^[4][7]

Bacteria are single-celled microorganisms that are very diverse, with a variety of shapes and features. They have the ability to live in almost any environment, including inside the human body. Not all bacteria cause infections. Those that do cause infection are called pathogenic bacteria. Many types of bacteria live on the skin or in the gut and are actually helpful, supporting digestion and overall health. However, when harmful bacteria enter the body or when normally harmless bacteria multiply uncontrollably in the wrong place, they can cause disease.^[7][8]

Fungi live on the body and in the environment around us. When fungi get overgrown or get into places in the body where they shouldn’t be, they can make people sick. Common fungal infections can affect the skin, nails, and internal organs.^[2]

Parasites are organisms that live off the bodies of other organisms. They can enter the body through contaminated food or water, insect bites, or direct contact with infected individuals or animals. Parasitic infections can affect various parts of the body and cause a range of symptoms.^[2]

Prions are faulty proteins that make other proteins become faulty too. Prion diseases, also called transmissible spongiform encephalopathies, are an extremely rare cause of infectious diseases. These conditions are caused by abnormal proteins rather than living organisms.^[2]

How Infections Spread

For an infection to occur, germs must transmit to a person from a source, enter their body, invade tissues, multiply, and cause a reaction. Understanding how germs are spread and taking action to break the chain of infection is critical for preventing disease.^[16]

Bacterial infections happen when harmful bacteria enter the body and increase in number. They may also occur when bacteria that usually live on the skin or inside the body start to multiply uncontrollably, causing an infection. There are several ways bacteria and other pathogens can enter the body. Through a wound, cut, or bite that breaks the skin. By eating or drinking contaminated food or water. Through breathing in droplets from an infected person when they cough, sneeze, or breathe nearby. By touching contaminated surfaces and then touching the eyes, nose, or mouth.^[8]

Different infections spread in different ways. Some spread from person to person when someone coughs or sneezes. Close contact with another person, such as kissing or sexual contact, can transmit infections. Sharing utensils or cups with other people can pass germs along. Pathogens can live on surfaces like doorknobs, phones, and countertops. Contact with feces from a person or animal with an infectious disease can spread illness. Bug or animal bites can transmit infections. Contaminated or improperly prepared food or water is another route. Some infections pass from a pregnant woman to the fetus or newborn. Blood transfusions, organ or tissue transplants, or other medical procedures can occasionally transmit pathogens.^[2]

Floods or environmental disruptions can bring harmful bacteria from soil to the surface. Flood water can come from weather events such as storms or cyclones. Touching items affected by flooding can increase the risk of bacterial infection. In areas affected by flooding, avoiding contact with flood water or any contaminated water and practicing good hand hygiene is essential.^[8]

Risk Factors

Anyone can get an infectious disease. However, certain people are at an increased risk of developing infections and experiencing more severe complications from them.^[2]

People with weakened immune systems face higher risk. This includes individuals with cancer, HIV infection, or those who have received organ transplants. These conditions can decrease the immune system’s ability to fight infection. People who take certain medications such as antibiotics, steroids, and some cancer-fighting medications may have a decreased ability to fight off infections. Those receiving treatments or procedures such as urinary catheters, tubes, and surgery face additional risk because these provide extra ways for germs to enter the body.^[16]

Age plays a significant role in susceptibility to infections. Infants, children, and older adults are at higher risk compared to healthy adults. The very young have immune systems that are still developing, while older adults may have weakening immune defenses as part of the aging process.^[2][18]

People who have underlying medical conditions such as diabetes, cancer, heart disease, or lung disease are more susceptible to infections. These chronic conditions can compromise the body’s natural defenses and make it harder to fight off pathogens when they enter the body.^[16]

There are many factors that affect the body’s ability to defend against infection. When someone becomes seriously ill and their body is fighting to recover, they become more vulnerable. People recovering from sepsis, for example, are at risk of dying from a second infection because of their weakened condition, even after they overcome the first infection that caused the sepsis.^[18]

Individuals who work in healthcare settings or live in crowded conditions like nursing homes or dormitories may be at higher risk of exposure to infectious diseases. Travel to certain regions where specific infectious diseases are common can also increase risk. Not being vaccinated against preventable diseases is another important risk factor that can be addressed through immunization.^[2]

Symptoms

Symptoms of infectious diseases vary depending on the cause of the infection. Each infectious disease has its own specific symptoms based on which part of the body is affected and what type of pathogen is involved. However, there are some general symptoms that many infections share.^[5]

Common symptoms that occur with many types of infections include fever, which is the body’s natural response to fighting off pathogens. Many people experience tiredness or fatigue, as the body uses energy to combat the infection. A faster heartbeat can occur as the body works harder during illness. Muscle aches and general body pains are frequent complaints. Some people develop headaches during infections.^[5]

Respiratory symptoms are common with infections that affect the airways and lungs. These can include coughing that may last for a week or more, sometimes producing yellow-green mucus or blood. Difficulty breathing or shortness of breath may occur with more severe respiratory infections. A sore throat often accompanies infections of the upper respiratory tract.^[5]

Digestive symptoms such as diarrhea, nausea, and vomiting can occur with infections that affect the gastrointestinal system. These symptoms can also accompany other types of infections as the body responds to illness. Some people experience a lack of appetite or find that food doesn’t taste normal when they’re sick.^[8][19]

Skin changes or rashes can develop with certain infections. The skin may become dry and itchy, and in some cases may peel. Some infections cause specific types of rashes that help healthcare providers identify the illness. Wounds, cuts, or surgical sites may show signs of infection like redness, warmth, swelling, or discharge of fluid.^[8][19]

Night sweats can occur with some infections as the body’s temperature regulation is affected. Chills and feeling generally unwell are also common experiences during infections. Some people develop confusion or disorientation, especially with more severe infections or in vulnerable populations like the elderly.^[5][18]

It’s important to seek medical care immediately if certain symptoms develop. These include being bitten by an animal, having trouble breathing, coughing for more than a week or coughing up colored mucus or blood, having a severe headache with fever, or developing a widespread rash that comes with fever, leaks fluid, or feels painful or warm. These symptoms could indicate a serious infection that requires prompt medical attention.^[5]

Prevention

Preventing infections is always better than treating them. There are many strategies available to help protect people from getting infected and to prevent the spread of disease to others. Some are simple steps that individuals can take in their daily lives, while others are broader public health measures.^[12]

Vaccination

Vaccines are biological preparations that improve immunity to particular diseases. A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe or its toxins. The agent stimulates the body’s immune system to recognize it as foreign, destroy it, and remember it, so that the immune system can more easily identify and destroy any of these microorganisms it encounters later.^[12]

Many infectious diseases such as measles and chickenpox can be prevented by vaccines. Other illnesses like COVID-19 might make people less sick if they get vaccinated. Because vaccines are widely used in many countries, diseases that were once common are now rare or well controlled. Examples include polio, measles, diphtheria, whooping cough, mumps, tetanus, and certain forms of meningitis.^[5][12]

Vaccinated people produce antibodies that neutralize disease-causing viruses or bacteria. They are much less likely to become infected and transmit those germs to others. Even people who have not been vaccinated may be protected by the immunity of the “herd,” because the vaccinated people around them are not getting sick or transmitting the infection. The higher the proportion of vaccinated people in a community, the lower the likelihood that a susceptible person will come into contact with an infectious individual, leading to greater herd immunity.^[12]

Hand Hygiene

Hand hygiene is crucial to preventing the spread of germs and is one of the most effective ways to avoid getting sick. People should wash their hands often with warm soap and water for at least 20 seconds. Hands should always be washed after using the bathroom, blowing the nose, coughing, or sneezing. It’s also important to wash hands before preparing or eating food, and after handling raw meat, touching animals, or caring for someone who is sick.^[17][22]

When washing hands, it’s important to wet them thoroughly, lather up with soap, and rub it into the palms and backs of the hands and wrists. Special attention should be paid to cleaning fingertips, under nails, and between fingers. After rinsing under running water, hands should be dried thoroughly. If soap and water aren’t available, using alcohol-based hand sanitizer with at least 60 percent alcohol can help reduce the spread of infection.^[17][23]

Healthcare workers and patients should ensure that everyone cleans their hands before touching patients or before medical procedures. Patients should not hesitate to remind healthcare workers and visitors to clean their hands. This simple step can significantly reduce the risk of healthcare-associated infections.^[21]

Respiratory Hygiene

Covering coughs and sneezes is important for preventing the spread of respiratory infections. People should cover their mouth and nose with a tissue when they sneeze or cough, then dispose of the tissue immediately. If no tissue is available, coughing or sneezing into the elbow rather than into the hands helps prevent spreading germs. This is especially important because respiratory infections spread easily between people through droplets released when someone coughs or sneezes.^[17][9]

When someone is sick, they should avoid touching their face, particularly the eyes, nose, and mouth, without washing their hands first. Most upper respiratory infection viruses are spread through mucosal membranes like the eyes and nose. These viruses need a place to enter the body, so avoiding touching these areas with unwashed hands reduces the risk of infection.^[22]

Food Safety

Although most cases of food-borne infection are not dangerous, some can lead to serious medical conditions. People can prevent infections by food-borne pathogens by preparing and storing foods safely. All meat, poultry, fish, fruits, and vegetables should be rinsed under running water before cooking or serving. Hands should be washed with soap and water before and after handling raw meat.^[17]

Raw foods and cooked foods should be kept separate. The same utensils or cutting boards shouldn’t be used with cooked meat that were used to prepare raw meat without washing between uses. Food should be cooked to proper temperatures to kill harmful germs. Eating plenty of fresh fruits and vegetables ensures a balanced diet with plenty of vitamins and minerals that support immune function.^[17][22]

Healthy Lifestyle Habits

Getting enough sleep is crucial for maintaining a strong immune system. A lack of sleep may compromise immune defenses, making people more susceptible to contracting viruses. Most people need an average of seven to eight hours of sleep a night. Toddlers and young children require 10 to 12 hours of sleep every day. Sticking to a regular bedtime and disconnecting from electronic devices well before sleep can help ensure quality rest.^[22][23]

Staying hydrated helps the immune system function properly. Drinking approximately eight glasses of water a day is a good goal. When sick, lots of clear fluids like water and herbal tea help flush the body, soothe the throat, regulate temperature, and keep mucus secretions thin. Coffee, soda, and alcohol should be limited as they are often high in sugar and can cause dehydration.^[23]

A balanced healthy diet supports immune function. Eating plenty of fresh fruits and vegetables provides vitamins and minerals that help the body fight off infections. Citrus fruits provide vitamin C, and vegetables like carrots and sweet potatoes are rich in beta-carotene, which converts to vitamin A and helps mucous membranes function properly. Fried and processed foods or fast food meals should be limited. Sugar should be considered a treat to have in very small quantities.^[22][23]

Not smoking or quitting tobacco use is important for reducing infection risk. Smoking increases the risk of contracting infections and also makes most cold symptoms worse. It doesn’t matter how old someone is or how long they’ve been smoking – quitting at any time will improve health.^[23]

Incorporating exercise into daily life helps maintain overall health and supports immune function. Regular physical activity can strengthen the body’s natural defenses against infections.^[23]

Avoiding Exposure

People should avoid close contact with individuals who are sick when possible. When visiting or caring for a sick person, extra attention should be paid to hand hygiene. Not sharing dishes, glasses, or eating utensils helps prevent the spread of germs. Direct contact with napkins, tissues, handkerchiefs, or similar items used by others should be avoided.^[17]

All cuts should be washed and bandaged promptly. Any serious cut or animal or human bite should be examined by a healthcare provider. Healing wounds or blemishes shouldn’t be picked at, and pimples shouldn’t be squeezed, as these actions can introduce bacteria and cause infections.^[17]

When traveling to areas with a high risk of diseases from insect bites, people should use EPA-registered insect repellent and wear long pants, shirts, and socks. Being aware of flood risks and avoiding contact with flood water or contaminated water can prevent exposure to harmful bacteria brought to the surface during environmental disruptions.^[2][8]

Pathophysiology

The pathophysiology of infection involves understanding how pathogens interact with the body and cause disease. This includes the processes of colonization, the development of disease, and the body’s response to infectious agents.^[1]

Colonization

Colonization occurs when germs are present on or in the body but do not cause symptoms of an infection. A person who is colonized has microorganisms living on their skin or in their body without becoming sick. However, colonized people can still transmit the germs they carry to others. This is important because it means someone can spread an infectious disease even when they don’t feel ill themselves.^[16]

Many microorganisms occur naturally and are present everywhere in the environment. Some microorganisms live on the skin, in the nasopharynx, and in the gastrointestinal tract but don’t become an infection unless the host becomes susceptible. These microorganisms make up the normal flora that every person carries. They typically don’t cause problems and may even be beneficial, helping with digestion and protecting against harmful germs.^[7]

Development of Disease

For an infection to develop into disease, several steps must occur. Germs must first transmit to a person from a source. Sources can include other people such as patients, healthcare workers, and visitors. They can be surfaces in care areas like bed rails, medical equipment, countertops, and tables. Wet environments like cooling towers, faucets, and sinks can harbor pathogens. Medical devices such as catheters and intravenous lines can serve as sources. Even dust or decaying debris from construction or water leaks can contain infectious agents.^[16]

Once transmitted, pathogens must enter the body through some route. They can enter through broken skin from wounds or bites. They can be inhaled into the respiratory system. They can be ingested with contaminated food or water. They can enter through mucous membranes in the eyes, nose, or mouth.^[2]

After entering the body, pathogens must invade tissues and multiply. Different types of pathogens use different strategies. Viruses invade the body’s cells and use the components of those cells to replicate. After the virus replication cycle is complete, new viruses are released into the body, causing damage or destruction of the host’s cells. Some viruses can remain dormant in the body and reactivate under certain circumstances.^[4]

Bacteria multiply by reproducing on their own. They either kill cells directly or produce toxins (poisons) that damage cells and tissues. Many bacteria give off toxins that can damage cells and make people sick. The multiplication of bacteria and the release of toxins trigger symptoms and illness.^[14]

Immune Response

When pathogens enter the body and begin to cause disease, the body reacts with an immune response. Hosts fight infections using their immune systems. Mammalian hosts, including humans, react to infections with an innate response, often involving inflammation, followed by an adaptive response.^[1]

The immune system is an effective barrier against infectious agents. It recognizes foreign invaders and works to eliminate them from the body. Many of the symptoms people experience during an infection occur due to the immune system’s response to the pathogen. Fever, for example, is the body’s way of creating an environment less favorable for pathogen growth while enhancing immune function.^[4]

However, pathogens may sometimes overwhelm the immune system’s ability to fight them off. When the infection is particularly severe or the immune system is weakened, the body may not be able to control the infection on its own. In these cases, medical treatment becomes necessary to help the body overcome the infection.^[4]

In some cases, the body’s response to infection can itself cause problems. Sepsis happens when the body’s response to an infection spirals out of control. The body releases molecules into the blood called cytokines to fight the infection, but these molecules then trigger a chain reaction. This can cause damage to blood vessels and lead to very low blood pressure. Without normal blood flow, the body can’t get enough oxygen, and multiple organs can fail.^[18]

Research has found that during sepsis, the traditional understanding that the body responds too strongly is only part of the story. The body also makes too few of some important types of immune cells. This makes it hard for the body to effectively fight the infection that first triggered sepsis and causes additional damage. It also makes people more vulnerable to other germs, which is why survivors of sepsis are at risk of developing secondary infections.^[18]