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"I think we're learning every day about the virus and how it interacts with us as human hosts," Birx, the White House coronavirus response coordinator, said during an appearance on Fox News' "Justice with Judge Jeanine."
"And that's been very important, to constantly be triangulating data," she added. "I think we underestimated, very early on, the number of asymptomatic cases. And I think we're really beginning to understand there are people that get infected -- that those symptoms are so low-grade that they don't even know that they're infected.
"And we're beginning to see that with the New York studies of their zero-antibody studies."
Host Jeanine Pirro asked Birx whether deaths should be attributed to the virus even when patients have preexisting conditions that place them at high risk.
"This is a very complex virus, and I want to be very clear," Birx responded. "It's highly transmissible, very infectious, and a lot of people have become infected. And so I do believe that a lot of the diseases we're seeing in the hospital right now --.yes, they may have preexisting conditions but those preexisting conditions are resulting in them having a much more serious course when they're infected with this virus."
"This is a very complex virus, and I want to be very clear: It's highly transmissible, very infectious, and a lot of people have become infected."
Birx warned that as some states begin to reopen, more people who have preexisting conditions could be affected by the virus.
"We have to remember that in America, we have a lot of people -- even young people -- with diabetes and asthma and hypertension," Birx said. "And so protecting them really becomes very critical because they can have a very difficult course."
"And then the bottom line of all of this is: Every individual is still taking responsibility of washing their hands, not touching their face," Birx added. "And when you can't social distance to make sure that they're wearing a mask -- because you wear a mask to protect the other person."