The head of the Centers for Disease Control told Congress on Wednesday that preliminary data from nationwide testing shows that over 90 percent of Americans have not contracted the Coronavirus, as top federal health officials continued to implore Americans to take precautions in order to slow the virus outbreak.

“More than 90 percent of the population remains susceptible,” Dr. Robert Redfield told a Senate hearing, saying serology tests have shown clear trends in the spread of the virus.

“It varies in different geographic parts from states that have less than one percent with evidence of previous infection, to some that have more than 15, 20 and one as high as 24 percent,” Redfield added, saying the results would be released in the next week.

Redfield said the highest growing group of people getting infected with the virus are in the 18-25 year age range, which now represent over one-quarter of all new virus cases.

“They are major contributors to the spread of COVID-19,” Redfield said, noting that while they may not suffer major medical problems, they can easily be responsible for giving the virus to others, especially older family members.

Asked about virus outbreaks on college campuses, Dr. Anthony Fauci said the best option was to keep those young people in the college community, and not send them home, where they could further spread the virus.

“Do not send them home to their community, because of the likelihood of then re-seeding infection in the community,” Fauci testified.

The hearing came as Brown University put a new tracking website online focused on virus outbreaks in schools, with initial data showing less of a problem than many experts had feared.

At the hearing, Fauci, Redfield, and other health experts again all but pleaded with Americans to embrace public health measures as a way to both fight the virus, and help stem its spread while waiting on a vaccine.

“Wear a mask. Maintain social distance,” said Redfield. “Be smart about crowds.”

In his testimony, Fauci said early reviews of the health impacts of the virus - even on those who have recovered - have been unsettling.

“We do not completely understand the nature of this illness,” said Fauci, rattling off a series of problems for people who had the virus, including ‘a disturbing number of individuals’ with inflammation of the heart.