Health officials reported on Tuesday that there has been a spike in infections of a highly contagious seasonal respiratory virus in the U.S., which poses significant risks particularly for pregnant women and individuals with blood disorders.

Parvovirus B19 is spread through droplets from infected individuals who may show symptoms or may not display any outward signs of illness. The virus seems to be on the rise this summer, according to data from blood plasma samples, anecdotal evidence, and various observations, leading the U.S. Centers for Disease Control and Prevention (CDC) to alert health care providers, agencies, and the public.

In its notification, the CDC indicated that the percentage of individuals testing positive for antibodies—an indication of recent infection—rose across all age groups. The rate increased from under 3% showing immunity in 2022 to 10% in June 2024. Children aged 5 to 9 saw the largest increase. Among plasma donors, the detection of the virus’s DNA surged from 1.5% in December to 19.9% in June. The CDC has also noted a rise in complications reported in pregnant women and those with sickle cell disease. Furthermore, over a dozen European nations encountered “unusually high case counts” of parvovirus B19 during the first quarter of 2024, according to the CDC’s alert.

Parvovirus B19 is extremely contagious, with around half of those exposed in one household likely getting infected, as per the CDC. Similar infection rates have been recorded in schools during outbreaks. While most cases are mild, symptoms can include fever, headache, cough, and sore throat. In children, a fever or flu-like symptoms may occur first, followed by a distinctive reddish “slapped cheek” rash. Adults can experience painful and swollen joints that generally last between one and three weeks.

For people with blood disorders or weakened immune systems, parvovirus B19 can lead to severe decreases in blood counts, or anemia. Those particularly vulnerable include individuals with leukemia, organ transplant recipients, people living with HIV, and those with conditions like sickle cell disease and thalassemia. Severe consequences are rare but can encompass myocarditis (inflammation of the heart), encephalitis (swelling of the brain), and hepatitis.

In pregnant individuals, there is a risk of the virus transmitting to the fetus, which in rare cases can result in miscarriage.

The U.S. does not conduct routine tracking of parvovirus B19, nor is it a reportable disease. There is also no vaccine or specific treatment available. According to the Mayo Clinic, past infections typically confer lifelong immunity. To reduce the risk of infection, individuals are advised to wash their hands frequently, avoid touching their faces, and refrain from sharing food. Wearing masks can also help limit the spread, similar to precautions for other respiratory illnesses.