COVID-19 continues to be a confusing medical mystery. 

On Tuesday, CNBC reported that a 25-year-old Nevada man contracted the coronavirus on two separate occasions. This is the first reported case in the United States when a patient was re-infected with COVID-19 and only the fifth known case globally. 

To make matters more mysterious, the Washoe County resident had no known immune disorders or history of significant underlying conditions which normally makes a person more susceptible to the virus. Per a study in the Lancet Infectious Diseases journal, the man's second bout with COVID-19 took was more severe than the first time he fought the virus, leading to a hospital stay. 

Although he has since recovered, his multiple infections with the virus sparks concern about the possibility of becoming immune to the illness. Initially, it was assumed that the second coronavirus infection would be milder than the first. But, the Nevada man proved that this will not always be the case. 

"It means that it is possible to get reinfected, that’s all it really tells us," associate professor in cellular microbiology at the University of Reading, Dr. Simon Clarke, said when discussing the case with CNBC. "It doesn’t tell us that protective immunity is impossible. It is worth remembering that this might be just one of a very small handful of reinfections, it might be very rare, or it might be one of the very first few we are going to see a lot more of given time."

Reports of second contractions in Hong Kong, the Netherlands, and Belgium were all less serious than the first bout. But one case in Ecuador was similar to the Nevada case but it didn't require hospitalization. 

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