A virus many people associate with sick puppies is once again spreading across parts of the U.S., but this outbreak involves a completely different virus that infects humans.
Human parvovirus B19, the virus behind “fifth disease” or the so-called “slapped cheek” rash in children, has seen elevated activity in the U.S. since 2024, according to the Centers for Disease Control and Prevention (CDC). Wastewater surveillance from 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 southeastern 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 illness can sometimes lead to more serious complications in pregnant people and immunocompromised individuals.
Despite the name, human parvovirus B19 is not the same virus that causes canine parvovirus in dogs. According to the National Library of Medicine, parvovirus B19 infects only humans and cannot spread between people and pets.
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What Is Human Parvovirus B19?
Unlike canine parvovirus, which mainly attacks the digestive system in dogs and is known for causing severe 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 starts like a mild flu, with symptoms such as fever, headache, fatigue, and muscle aches before progressing into 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 at all.
People are usually most contagious before the rash appears, during the virus’s early flu-like stage. Once the rash develops, 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 red blood cell production.
Infections during the first half of pregnancy may also carry a higher risk of complications because the virus can sometimes cross the placenta and infect the fetus.
Why Is Human Parvovirus B19 Spreading Again?
Parvovirus B19 most commonly affects school-aged children between 5 and 15 years old, 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 childcare settings before families realize what is circulating.
Mini-outbreaks tend to appear in cycles every three to four years, especially during late winter, spring, and early summer.
Health officials across both the U.S. and Europe have seen parvovirus activity rise substantially since the COVID-19 pandemic. According to the National Library of Medicine, years of masking, school closures, and reduced social interaction disrupted the normal circulation of many childhood viruses. As those precautions faded, more children lacked recent exposure and immunity, making it easier for viruses like parvovirus B19 to spread again.
The CDC reports that activity increased significantly across the U.S. during 2024 and remained elevated into 2025, while wastewater surveillance continues to detect the virus in some communities in 2026.
How To Avoid Human Parvovirus B19?
There is currently no vaccine for human parvovirus B19. Because people are often contagious before symptoms appear, avoiding exposure entirely can be difficult once the virus begins circulating in schools and communities.
Still, experts say habits that reduce the spread of respiratory viruses, including handwashing, covering coughs and sneezes, and avoiding close contact with sick individuals, will help lower the risk of infection.
This article is not offering medical advice and should be used for informational purposes only.
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from the National Library of Medicine: Parvovirus B19 Infection
- This article references information from the CDC: About Parvovirus B19
- This article references information from the CDC: Increase in Human Parvovirus B19 Activity in the United States
- This article references information from the CDC: Parvovirus B19
- This article references information from the Mayo Clinic: Parvovirus infection
- This article references information from WastewaterSCAN: WastewaterSCAN Dashboard
