The United States "could be in for a bad fall" if researchers don't find an effective treatment to fight the coronavirus by then, White House health advisor Dr. Anthony Fauci said Tuesday.

The virus will certainly make a comeback in the U.S. even as cases begin to stabilize, Fauci, director of the National Institute of Allergy and Infectious Disease, said during an interview with The Economic Club of Washington, D.C.

Covid-19 is "not going to disappear from the planet," he said, adding infectious disease experts are learning about how the virus behaves by watching emerging outbreaks in other regions such as southern Africa that are starting to enter their colder seasons.

"In my mind, it's inevitable that we will have a return of the virus, or maybe even that it never went away," he said.

Fauci also warned against states reopening businesses prematurely, saying it could cause "a rebound to get us right back in the same boat that we were in a few weeks ago."

The virus, which emerged in Wuhan, China, almost four months ago, has sickened more than 3 million people worldwide and killed at least 212,000 as of Tuesday morning, according to data compiled by Johns Hopkins University. Roughly a third of the global cases are in the United States, making it the worst outbreak in the world.

U.S. officials say that they are preparing to battle two bad viruses circulating at the same time as the coronavirus outbreak runs into flu season next fall and winter.

Fauci said Tuesday that he is "cautiously optimistic" researchers can develop a vaccine to prevent Covid-19, but added nothing is ever "a guarantee."