Parvovirus Is Spreading Across Northern California — but Many Still Think of Parvo as a Dog Disease
A virus that many people associate with sick puppies is spreading again in parts of the United States, but this outbreak involves a completely different virus that infects humans.
Human parvovirus B19, the virus that causes “fifth disease” or the so-called “slap rash” in children, has seen high activity in the United States since 2024, according to the Centers for Disease Control and Prevention (CDC). Wastewater monitoring by WastewaterSCAN has also detected the virus in communities across the country, including moderate levels recently reported in parts of Northern California near Sacramento, San Jose, Palo Alto, Napa, Davis and southeast San Francisco.
The virus spreads primarily through respiratory droplets released when infected people cough or sneeze. Most infections are mild, especially in children, but the disease can sometimes cause more serious complications in pregnant people and people who are immunocompromised.
Despite its name, human parvovirus B19 is not the same virus that causes canine parvovirus in dogs. According to the National Library of Medicine, parvovirus B19 only infects humans and cannot spread between humans and pets.
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What is human parvovirus B19?
Unlike canine parvovirus, which primarily attacks the digestive system in dogs and is known to cause serious gastrointestinal illness in puppies, human parvovirus B19 targets immature red blood cell precursor cells in the bone marrow, according to the National Library of Medicine.
For many people, the infection begins as a mild flu, with symptoms such as fever, headache, fatigue, and muscle aches before progressing to more recognizable signs of illness. Children may later develop the characteristic bright facial rash associated with fifth disease, while adults are more likely to experience joint pain and swelling, particularly in the hands, wrists, knees and ankles – sometimes without ever developing the classic rash.
People are generally most contagious before the rash appears, early in the flu stage of the virus. Once the rash appears, most people are no longer considered contagious.
Most healthy people recover without complications. However, the virus can become more dangerous in people with weakened immune systems or certain blood disorders because it temporarily disrupts the production of red blood cells.
Infections occurring during the first half of pregnancy may also carry a higher risk of complications, as the virus can sometimes cross the placenta and infect the fetus.
Why is human parvovirus B19 spreading again?
Parvovirus B19 most commonly affects school-age children ages 5 to 15, according to the National Library of Medicine.
Like many respiratory viruses, it spreads easily through coughing, sneezing, saliva and close contact. Because children are often contagious before the telltale rash appears, outbreaks can spread quietly through schools and daycares before families realize what’s going around.
Mini-outbreaks tend to appear in cycles every three to four years, particularly in late winter, spring and early summer.
Health officials in the United States and Europe have seen a substantial increase in parvovirus activity since the COVID-19 pandemic. Years of masking, school closures and reduced social interactions have disrupted the normal circulation of many childhood viruses, according to the National Library of Medicine. As these precautions faded, more children lacked recent exposure and immunity, making it easier for viruses like parvovirus B19 to spread.
The CDC reports that activity increased significantly in the United States in 2024 and remained high through 2025, while wastewater monitoring continues to detect the virus in some communities into 2026.
How to avoid human parvovirus B19?
There is currently no vaccine against human parvovirus B19. Because people are often contagious before symptoms appear, it can be difficult to completely avoid exposure once the virus begins circulating in schools and communities.
Still, experts say habits that reduce the spread of respiratory viruses, including washing hands, covering coughs and sneezes, and avoiding close contact with sick people, will help reduce the risk of infection.
This article does not offer medical advice and should be used for informational purposes only.
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