Can You Get Coronavirus More Than Once?

Recovered patients who test positive again raise questions about the virus’s ability to reactivate

Over 100 people in South Korea who were considered to be cleared of Covid-19 have tested positive again. It isn’t clear why this is happening, but the unsettling discovery has prompted the World Health Organization (WHO) to begin an official investigation.

Several explanations for the positive test results have been proposed in the meantime, including the possibility that the tests were faulty or that recovered people might have been reinfected, Reuters reports. In a press briefing, Jeong Eun-kyeong, the director of the Korea Centers for Disease Control and Prevention (KCDC), said the SARS-CoV-2 virus that causes Covid-19 may have been “reactivated” rather than the patients being reinfected.

Reinfection and reactivation are two very different things. When a person is reinfected, they make a full recovery from their initial infection and then become infected with the virus again. Reactivation, meanwhile, is when a virus switches from the “latent,” or dormant, phase of its life cycle into a “lytic” phase, in which it starts making copies of itself again. Not all viruses are known to reactivate.

Philip Tierno Jr., PhD, a professor of microbiology and pathology at NYU Grossman School of Medicine, explained what we know right now about coronavirus and reactivation, as well as testing and reinfection, to the Medium coronavirus blog.

“Firstly,” he said in an email, coronaviruses “are not the type of virus group that reactivates like Epstein-Barr or Ebola.” The Epstein-Barr Virus (EBV), which is extremely common and best known for its ability to cause mono, becomes latent in the body after an infection and can reactivate, though it does not always cause symptoms, according to the CDC. Ebola has also been known to become latent and then reactivate. Another virus with this ability is the varicella-zoster virus (VZV), which causes chicken pox and, when it reactivates, shingles. Neither EBV, Ebola, nor VZV belongs to the class of viruses known as coronaviruses, which SARS-CoV-2 belongs to.

Myoung-don Oh, a professor of internal medicine at Seoul National University and a member of the WHO’s Strategic and Technical Advisory Group for Infectious Hazards, told TIME that reactivation is not as likely an explanation as faulty tests.