How long could I be contagious before a positive virus test? Studies have shown that people may be contagious for about two days before developing COVID-19 symptoms. Experts say right before developing symptoms is when people are likely the most contagious. People who never develop symptoms can spread infection, too. That’s a problem because many people would never seek testing unless they developed symptoms or knew they’d been exposed. But there’s a more complicated part to this question: What if someone knows they were exposed but their virus test comes back negative — could they still be contagious? Maybe.