Fifth disease is caused by a virus called parvovirus B19. This is a common childhood infection that spreads through respiratory droplets, much like a cold, and produces the distinctive “slapped cheek” rash that most parents recognize. The virus specifically targets cells in the bone marrow that are responsible for producing red blood cells, which is why it can occasionally cause complications beyond the familiar rash.
The Virus Behind Fifth Disease
Parvovirus B19 is a small, single-stranded DNA virus that infects only humans. (It’s unrelated to the parvovirus that affects dogs.) The virus has a very specific target in the body: it latches onto a receptor called the P antigen, found on the surface of red blood cell precursors in the bone marrow. These precursor cells are the young cells that eventually mature into the red blood cells circulating in your bloodstream.
Once the virus attaches, it enters the cell and hijacks the cell’s own machinery to copy its DNA. The virus can only replicate when the host cell is actively dividing, so it depends on the bone marrow environment where red blood cell production is constant. This takeover temporarily halts the production of new red blood cells. In a healthy person, this pause is brief and barely noticeable because existing red blood cells live for about 120 days. The body catches up quickly once the immune system clears the virus.
How It Spreads
Parvovirus B19 spreads in three main ways: through respiratory particles (coughs and sneezes), through blood or blood products, and from a pregnant person to their baby during pregnancy. Respiratory transmission is by far the most common route, especially among children in schools and daycare settings.
The tricky part about containing fifth disease is the timing of contagiousness. You’re most infectious during the first few days of symptoms, which at that point look like a generic cold: mild fever, runny nose, headache. By the time the telltale rash appears on the cheeks, you’re typically no longer contagious. This means most people spread the virus before anyone realizes it’s fifth disease. Once the rash shows up, it’s generally safe to return to school or work.
Why Symptoms Look Different in Kids and Adults
In children, fifth disease usually follows a predictable pattern. After an incubation period of about one to two weeks, a child develops mild cold-like symptoms. A few days later, the bright red “slapped cheek” rash appears on the face, sometimes followed by a lacy, net-like rash on the trunk, arms, and legs. The rash can come and go for several weeks, sometimes flaring with heat, sunlight, or exercise, but the child generally feels fine by this point.
Adults often have a different experience. While they may or may not develop a rash, they’re much more likely to develop joint pain and swelling, particularly in the hands, wrists, and knees. This joint involvement is most common in women. The pain usually resolves within one to three weeks, but in some cases it can linger for months or even longer.
Why It’s Riskier for Some People
Because parvovirus B19 specifically destroys the precursor cells that produce red blood cells, it can cause serious problems for people whose bodies already break down red blood cells faster than normal. People with sickle cell disease, hereditary spherocytosis, or other conditions involving fragile red blood cells depend on a constant, high rate of new cell production. When the virus halts that production, even briefly, the combination of ongoing cell destruction and no new cells being made can trigger a sudden, severe drop in red blood cell counts. This is called a transient aplastic crisis, and it can be life-threatening, sometimes requiring emergency blood transfusions.
People with weakened immune systems face a different problem. Their bodies may not be able to clear the virus effectively, allowing it to persist in the bone marrow and cause chronic anemia. These individuals may also remain contagious for much longer than the typical patient.
Risks During Pregnancy
When a pregnant person contracts parvovirus B19, the virus can cross the placenta. The rate of transmission from mother to fetus ranges from 17% to 33%. The primary concern is that the virus can destroy red blood cell precursors in the developing baby, leading to severe fetal anemia and a dangerous condition involving fluid buildup called hydrops fetalis.
The risk depends heavily on timing. Infection during the first trimester carries the highest risk of hydrops, estimated at 5% to 10%. Between 13 and 20 weeks, that risk drops to 5% or less. After 20 weeks, it falls to around 1% or less. Most pregnant people who are infected with parvovirus B19 go on to have healthy pregnancies, but monitoring is important when infection is confirmed.
How Fifth Disease Is Diagnosed
In children, diagnosis is often made by appearance alone. The slapped-cheek rash is distinctive enough that a doctor can usually identify it without lab work. When blood tests are needed, particularly for pregnant individuals, adults with joint symptoms, or people with blood disorders, doctors look for specific antibodies. The presence of IgM antibodies against parvovirus B19 is the most reliable marker of a recent or active infection, detected in about 97% of cases. These antibodies can remain in the blood for up to six months after infection. Viral DNA can also be detected in blood samples for up to two months after symptoms begin.
Once you’ve recovered from parvovirus B19 infection, you develop lifelong immunity. A blood test showing IgG antibodies without IgM means you were infected at some point in the past and are protected from future infection. By adulthood, roughly half of all people have antibodies from a previous infection, often one they never noticed.
Treatment and Recovery
For the vast majority of people, fifth disease requires no specific treatment. It’s a self-limiting infection, meaning the immune system clears it on its own. Comfort measures like rest, fluids, and over-the-counter pain relief for fever or joint discomfort are all that’s needed. The rash itself isn’t painful or itchy for most children, though it can occasionally cause mild itching.
Treatment becomes more involved for people who develop complications. Those experiencing a transient aplastic crisis typically need blood transfusions to support their red blood cell counts until the bone marrow recovers. For immunocompromised patients with chronic parvovirus infection, treatment with intravenous immunoglobulin (a concentrated dose of antibodies from donated blood) can help the body neutralize the virus and restore normal red blood cell production. Pregnant individuals with infected fetuses may require close ultrasound monitoring, and in severe cases, the baby may need an intrauterine blood transfusion.
There is no vaccine for parvovirus B19. Because the virus spreads through respiratory droplets before anyone knows it’s fifth disease, standard hygiene practices like frequent handwashing and covering coughs are the most practical prevention measures. Pregnant individuals and people with blood disorders or weakened immune systems should be especially cautious around anyone with cold-like symptoms during outbreaks.