CMV stands for cytomegalovirus, a common virus that belongs to the herpesvirus family. Over half of all adults have been infected with it by age 40, and most never realize it. For the vast majority of people, CMV causes no symptoms and no lasting health problems. It becomes a serious concern in two specific situations: during pregnancy, when it can pass to an unborn baby, and in people with weakened immune systems, such as organ transplant recipients or those with advanced HIV.
How CMV Spreads
CMV passes from person to person through body fluids: saliva, urine, blood, breast milk, semen, and vaginal fluids. Young children are frequent carriers, which is why parents and childcare workers are commonly exposed. Changing diapers, wiping a toddler’s nose, or sharing utensils with a young child are all routine ways the virus spreads. Sexual contact and organ or blood transfusions are other transmission routes.
The virus can also pass from a pregnant person to their developing baby through the placenta. This is called congenital CMV, and it’s the route that carries the most serious health consequences.
Why Most People Never Notice It
In healthy adults and children, a CMV infection is usually silent. Some people develop a mononucleosis-like illness with prolonged fever and mild liver inflammation, but this resolves on its own. In the United States, roughly 30% of children have been exposed by age 5, and the numbers climb steadily from there. Once infected, you carry the virus for life.
Latency and Reactivation
Like other herpesviruses (the same family that includes chickenpox and cold sores), CMV doesn’t leave the body after the initial infection. Instead, it goes dormant inside certain immune cells, particularly early-stage blood cells in the bone marrow. The virus essentially switches off its own genes through a chemical silencing process, hiding from the immune system indefinitely.
Reactivation happens when the immune system is disrupted. Inflammation, other infections, serious illness, or medications that suppress immunity can flip the switch back on, allowing the virus to start replicating again. This is why CMV is a major concern after organ transplants, when patients take drugs that deliberately suppress their immune response. In these situations, the reactivated virus can cause pneumonia, liver disease, and inflammation of the digestive tract or eyes.
CMV During Pregnancy
The biggest risk from CMV affects babies exposed before birth. When a pregnant person catches CMV for the first time during pregnancy (a primary infection), the transmission rate to the fetus is as high as 50%. If the mother was already carrying the virus before pregnancy and it reactivates, the transmission rate drops to less than 2%.
About 1 in 5 babies born with congenital CMV will develop birth defects or long-term health problems. Hearing loss is the most common complication and can appear even in babies who seem perfectly healthy at birth. It may start in one ear and spread to the other, or progress from mild to severe during the first two years of life, a critical window for language development. In some cases, hearing continues to worsen through the teenage years, affecting communication and social skills over time.
If you’re pregnant or planning to become pregnant, simple hygiene steps can lower your risk. Wash your hands thoroughly after changing diapers, avoid sharing food or utensils with young children, and don’t kiss toddlers on the mouth or near the eyes. These precautions matter most for people who haven’t already been exposed to CMV.
How CMV Is Diagnosed
CMV is diagnosed through blood tests that look for two types of antibodies your immune system produces in response to the virus. The first, called IgM, typically appears during an active or recent infection and fades within about three months. The second, IgG, develops later and stays positive for life, indicating you’ve been infected at some point.
Interpreting these results isn’t always straightforward. A positive IgM doesn’t guarantee a new infection. It can also show up during a reactivation or even as a false positive. When both IgM and IgG are present, doctors can run an additional test called an IgG avidity test. A low avidity score alongside a positive IgM points to a recent primary infection, while a high avidity score means the infection happened more than three to four months ago. This distinction matters most during pregnancy, where the timing of infection determines the level of risk to the baby.
For newborns, testing is typically done within the first few weeks of life using saliva or urine samples, since antibody results in infants can reflect the mother’s immune response rather than the baby’s own infection.
Treatment for CMV
Healthy people who catch CMV don’t need treatment. The infection resolves on its own, and symptoms, if any, are managed with rest and basic supportive care.
For high-risk groups, antiviral medications are available. Transplant recipients at risk for CMV disease typically receive preventive antiviral therapy starting shortly after their procedure, continuing for several months. For babies born with symptomatic congenital CMV affecting the brain or nervous system, antiviral treatment may help slow the progression of hearing loss and improve developmental outcomes, though it doesn’t eliminate the virus entirely.
No vaccine for CMV currently exists, making prevention through hygiene and awareness the primary strategy for those most vulnerable to its effects.