Cytomegalovirus infection is a widespread viral illness that most people will unknowingly encounter during their lifetime, yet it remains virtually silent in healthy individuals while posing serious risks to unborn babies and those with weakened immune defenses.
Epidemiology
Cytomegalovirus, commonly known as CMV, is remarkably common throughout the world. This virus affects people in every country and across all age groups, though most infected individuals remain completely unaware of their status. The spread of this infection varies considerably depending on where you live, your economic circumstances, and your ethnic background.[1]
In the United States and Canada, experts have found that approximately one in three children has already been infected by the time they reach five years old. As people age, their likelihood of having encountered the virus increases substantially. By age 40, about half of all adults have been infected with CMV at some point in their lives. The numbers continue to climb with advancing age, and by the time people reach 80 years old, up to 90 percent will have been infected.[2]
Worldwide, the rates of CMV infection show striking variation. Depending on the region studied, anywhere from 24 to 100 percent of the population carries antibodies to the virus, indicating past or current infection. This enormous range reflects differences in living conditions, population density, and public health infrastructure across different parts of the globe.[3]
When it comes to babies born with CMV, the numbers are also significant. Approximately one in every 200 babies in the United States is born with congenital CMV, which means they acquired the infection before birth. This makes CMV the most common infectious cause of birth defects in the country. Among these infected newborns, about one in ten will show signs of illness at birth.[4]
The risk of CMV is particularly elevated among certain groups. Young children, especially those under five, show high infection rates. People who work in daycare settings or live with young children face increased exposure to the virus. Those with compromised immune systems, including individuals living with HIV/AIDS or those who have received organ or stem cell transplants, are at substantially higher risk for serious complications.[2]
Causes
Cytomegalovirus belongs to the Herpesviridae family, which is a large group of viruses that share certain characteristics. Specifically, CMV is also known as herpesvirus-5 or HHV-5. Like other members of this family, CMV is a double-stranded DNA virus, meaning it carries its genetic information in a specific molecular structure.[3]
The CMV virus has the distinction of containing the largest genome of any herpes virus. Its genetic material measures approximately 230 kilobases and contains around 200 genes that code for proteins. This extensive genetic library allows the virus to produce many different proteins, some of which help it evade the immune system and establish long-term residence in the body.[3]
One of the most important characteristics of CMV, shared with other herpes viruses, is its ability to establish lifelong latency in the body. After the initial infection occurs, the virus never completely leaves. Instead, it remains dormant, typically hiding inside certain white blood cells called monocytes. The virus uses sophisticated mechanisms to silence its own genes during this dormant phase, essentially going into hibernation while remaining present in the body.[3]
This latent virus can reactivate later, particularly when the immune system becomes weakened. When reactivation occurs, the virus begins multiplying again and can cause illness, especially in people whose immune defenses are compromised. Additionally, a person who has already had CMV can become infected again with a different strain of the virus, since there are multiple varieties circulating in human populations.[4]
Risk Factors
Several factors increase a person’s likelihood of contracting CMV or experiencing complications from the infection. Understanding these risk factors helps identify who needs to take extra precautions against this common virus.
Age plays a significant role in CMV exposure. Children under five years old have particularly high infection rates, with about one-third carrying the virus by their fifth birthday. Young children are efficient spreaders of CMV because the virus can remain in their bodily fluids for months after they become infected, and they typically have close physical contact with caregivers and other children.[2]
People who work with or live around young children face elevated risk. Daycare workers, preschool teachers, and parents of young children encounter the virus more frequently than others. The constant exposure to saliva, urine, and other body fluids during routine childcare activities creates opportunities for transmission.[4]
Living in crowded conditions increases the chance of CMV transmission. When many people share close quarters, the virus spreads more easily through the frequent exchange of bodily fluids that occurs during normal social interactions and daily activities.[2]
Having a weakened immune system dramatically increases both the risk of infection and the severity of illness. This applies to people living with HIV/AIDS, individuals undergoing chemotherapy or radiation therapy for cancer, and those taking medications that suppress immune function. Recipients of organ, stem cell, or bone marrow transplants face particularly high risk because they must take powerful immune-suppressing drugs to prevent rejection of the transplanted tissue. In these individuals, the virus can reactivate from its dormant state or cause severe illness if newly acquired.[1]
Sexual activity and intimate contact provide pathways for CMV transmission, since the virus can be present in semen and other fluids exchanged during sexual encounters. Blood transfusions and organ transplants can also transmit CMV from an infected donor to a recipient, though screening procedures help reduce this risk.[1]
Symptoms
The symptoms of cytomegalovirus infection vary dramatically depending on who becomes infected and the state of their immune system. In many cases, CMV causes no symptoms whatsoever, which is why the infection often goes unrecognized.
Most healthy people who contract CMV experience either no symptoms or such mild symptoms that they dismiss them as a common cold or flu. When symptoms do appear in otherwise healthy individuals, they typically include fever, extreme tiredness or fatigue, muscle aches, headache, and sore throat. Some people develop swollen glands, called lymph nodes, particularly in the neck. A skin rash or excessive sweating may also occur. These symptoms resemble infectious mononucleosis, an illness commonly caused by another herpes virus called Epstein-Barr virus. Occasionally, CMV can cause inflammation of the liver, a condition called hepatitis, resulting in yellowing of the skin and eyes.[1][2]
When symptoms do develop in healthy people, they typically resolve on their own within about three weeks without requiring specific medical treatment. The body’s immune system successfully controls the infection, though the virus remains dormant in the body indefinitely.[7]
The picture changes considerably for people with weakened immune systems. In these individuals, CMV can cause severe and potentially life-threatening illness. Symptoms depend on which organs the virus attacks. If it affects the lungs, symptoms include shortness of breath, dry cough, and difficulty breathing, a condition called CMV pneumonitis. When the virus infects the digestive system, it can cause diarrhea that may be bloody, abdominal pain, and difficulty or pain when swallowing due to ulcers in the mouth or esophagus. Eye involvement, particularly in people with advanced HIV, can lead to blurred vision, blind spots, or moving black spots called floaters in the visual field. This condition, called CMV retinitis, can progress to blindness if not treated. The virus may also affect the brain, causing confusion or other neurological symptoms.[2][6]
Babies born with congenital CMV show a wide range of possible symptoms, and many appear completely healthy at birth despite being infected. Among those who do show signs of infection at birth, symptoms can include premature delivery, low birth weight, yellowing of the skin and eyes (jaundice), an enlarged liver and spleen, a small head compared to body size (a condition called microcephaly), seizures, skin rashes that look like bruises, lung infection or pneumonia, and purple skin blotches.[5][4]
Even babies who appear healthy at birth may develop problems later. The most common long-term complication is hearing loss, which may be detected shortly after birth or may not appear until months or years later during childhood. Other potential long-term effects include vision problems, developmental delays, intellectual disability, learning difficulties, problems with coordination and muscle use, and seizures. Approximately one in five babies born with congenital CMV will experience birth defects or other lasting health problems.[4][5]
Prevention
Since there is currently no vaccine available to prevent CMV infection, prevention relies heavily on behavioral measures and good hygiene practices. These strategies are particularly important for pregnant women and those planning to become pregnant, as well as for people with weakened immune systems.
Hand washing represents the single most effective prevention measure. Washing hands thoroughly with soap and water is highly effective at removing the virus from the skin. This is especially important after changing diapers, helping young children use the toilet, wiping children’s noses, or touching children’s toys. The virus is present in the urine and saliva of infected children, sometimes for many months, so these routine caregiving activities create opportunities for transmission.[1][7]
Avoiding contact with saliva and urine from young children helps reduce exposure. This means not sharing food, eating utensils, drinking cups, or toothbrushes with young children. Avoid putting a child’s pacifier or bottle in your mouth. When kissing young children, aim for the forehead or cheek rather than the lips, where saliva exchange is more likely. These precautions are particularly crucial for pregnant women who work in childcare settings or have young children at home.[7][4]
Regularly washing toys, countertops, and other surfaces that come into contact with children’s urine or saliva helps eliminate the virus from the environment. This is particularly important in daycare centers and homes with multiple young children.[4]
After handling used tissues or diapers, avoid touching your face until you have thoroughly washed your hands. The virus can enter the body through the mucous membranes of the eyes, nose, and mouth, so preventing hand-to-face contact after potential exposure is an important protective measure.[1]
Sexual partners should be aware that CMV can be transmitted through sexual contact, as the virus may be present in semen and other bodily fluids. Using barrier methods during sexual activity can help reduce transmission risk.[1]
For people with weakened immune systems, avoiding unnecessary exposure to CMV is important. This includes being cautious about contact with young children and maintaining excellent hand hygiene. Healthcare providers may recommend preventive antiviral medications for certain high-risk individuals, particularly transplant recipients.[9]
While researchers are actively developing vaccines against CMV, none has yet been licensed for use. It will likely be several years before a CMV vaccine becomes available to the public.[20]
Pathophysiology
Understanding how CMV affects the body at a cellular and organ level helps explain why it causes such different effects in healthy versus immunocompromised individuals, and why it can be so devastating to developing fetuses.
After CMV enters the body, it initially infects cells at the site of entry, which might be the mucous membranes of the mouth, nose, throat, or genital tract. The virus then spreads through the bloodstream to various organs. CMV has a particular affinity for certain cell types, including epithelial cells that line organs and surfaces, endothelial cells that line blood vessels, smooth muscle cells, and various white blood cells.[3]
Once inside a cell, the virus hijacks the cell’s machinery to make copies of itself. The viral genetic material enters the cell’s nucleus, where it directs the production of new viral proteins and genetic material. The virus has evolved numerous strategies to evade detection and destruction by the immune system. Many of the proteins encoded by CMV’s large genome specifically interfere with immune responses. Some viral proteins hide infected cells from immune surveillance, while others interfere with inflammatory signals that would normally alert the immune system to the infection.[3]
In healthy individuals with normal immune function, the immune system mounts a vigorous response to CMV infection. Various components of immunity, including antibodies and specialized immune cells, work together to control the infection. While the immune system successfully limits viral replication and clears active infection from most tissues, it cannot completely eliminate the virus. CMV establishes latency primarily in monocytes and certain other white blood cells. During latency, the virus remains dormant with its genes turned off through specific molecular mechanisms. The infected cells harbor the viral genetic material but do not produce new virus particles.[3]
When the immune system becomes weakened, whether due to HIV infection, immunosuppressive medications, cancer treatments, or other causes, the delicate balance shifts in favor of the virus. CMV can reactivate from its latent state, or newly acquired infections can spread more aggressively because the immune system cannot mount an adequate response. The virus then replicates extensively, spreading through the blood (a condition called CMV DNAemia) and causing damage to multiple organs.[3]
In immunocompromised individuals, CMV can cause direct damage to organs through several mechanisms. The virus destroys infected cells as it replicates within them. It also triggers inflammation in affected tissues, and this inflammation itself causes additional tissue damage. In the lungs, this leads to pneumonitis with inflammation of the air sacs and difficulty breathing. In the gastrointestinal tract, the virus damages the lining of the esophagus, stomach, or intestines, causing ulcers and bleeding. In the eye, CMV infection of the retina destroys the light-sensing cells necessary for vision, potentially leading to blindness.[2]
During pregnancy, if a woman has an active CMV infection, the virus can cross the placenta and infect the developing fetus. The virus reaches the fetal bloodstream and spreads to various developing organs. Because the fetal immune system is still immature and developing, it cannot effectively control the infection. CMV can directly damage developing tissues and organs, particularly the brain, and may also interfere with normal developmental processes.[5]
In the developing fetal brain, CMV infection can cause several types of damage. The virus may destroy neurons directly, interfere with the formation of new neurons, or disrupt the migration of neurons to their proper locations in the developing brain. This can result in microcephaly (abnormally small head and brain), calcium deposits in brain tissue, and abnormal brain structure. Damage to developing structures in the inner ear can cause hearing loss, which may be present at birth or develop progressively during childhood. The earlier in pregnancy the infection occurs, generally the more severe the effects, because more of the crucial developmental processes are disrupted.[5]
The virus can persist in babies with congenital infection for months or years after birth, continuing to cause damage over time. This explains why some babies who seem unaffected at birth later develop hearing loss or other problems as they grow.[4]
