The scientific journal "The Lancet" published new information Monday about the Nevada man who contracted COVID-19 twice, and warned that even those who have recovered from the virus need to take precautions.

The 25-year-old patient had no known immune disorders or underlying conditions when he tested positive for the virus in April and again in June, according to the study. The patient was hospitalized during his second infection with severe symptoms, including dizziness, cough, nausea, and diarrhea.

He was released from the hospital and has since recovered, according to the journal.

Mark Pandori of the Nevada State Public Health Laboratory, the lead author of the study, said in a statement that while more research is needed, the data indicates that patients who recovered from an infection should still heed health warnings, such as avoiding crowds, wearing a mask and washing their hands.

"There are still many unknowns about SARS-CoV-2 infections and the immune system's response, but our findings signal that a previous SARS-CoV-2 infection may not necessarily protect against future infection," he said in a statement.

Experts say that people who recover from COVID-19 will likely have some form of immunity, but it's not clear if everyone develops immunity, or how long that immunity might last.

Reinfections are rare. The Nevada man is the only reported case in the U.S. so far of a patient contracting the virus twice. Four other cases of reinfection have been reported in Belgium, the Netherlands, Hong Kong, and Ecuador, according to the journal.

"So far, we've only seen a handful of reinfection cases, but that doesn't mean there aren't more, especially as many cases of COVID-19 are asymptomatic," Pandori said.

-ABC News' Sony Salzman contributed to this report.

Dr. Anthony Fauci again called on the Trump campaign to take down a recent political ad that used previous statements that he says were taken out of context, warning that the future use of his interviews and statements could "backfire" on the campaign.

Fauci appeared on CNN Monday and reiterated to Jake Tapper that he didn't give permission for his comments in a March Fox News interview, in which he complimented the overall efforts of federal public health officials, to be used in an ad in which it sounded like he was praising Trump personally.

"It's so clear that I'm not a political person. I have never either directly or indirectly endorsed a candidate," said Fauci, the director of the National Institute of Allergy and Infectious Diseases.

Despite Fauci's objections, Trump and the campaign officials said Sunday they would continue to run the ad.

When asked how he would react if the campaign ran more ads featuring his statements, Fauci said it would be "terrible" and "outrageous."

"That might actually come back to backfire on them; I hope they don't do that because that's, that would be kind of playing a game that we don't want to play," he said. "So I hope they reconsider that, if in fact they are indeed considering doing that."

Later Monday evening, Democratic presidential candidate Joe Biden tweeted a tongue-in-cheek video that cut together clips of Trump's speeches and interviews to make it look Trump apologized to Fauci.

"Donald Trump is running TV ads taking Dr. Fauci out of context and without his permission. So, here's a message from the President in his own words," Biden tweeted.

Biden later slammed Trump for going against Fauci's wishes during a campaign stop in Cincinnati, Ohio, saying, "Look, they use the ad knowing it's a lie. Can you remember any other major mainstream presidential candidate ever doing something like these things?"

Kentucky Gov. Andy Beshear, who is in quarantine after possible COVID-19 exposure, said he's doing well during a Monday news conference. "I feel great. My family feels great. We're trying to be really positive about this situation," he said.

The governor reminded the public to adhere to fundamental public health measures like mask wearing and social distancing. "Folks, wear it," he said of face coverings. "It's a requirement in Kentucky. The more you wear them, the less the virus will spread."

Beshear and his family are in quarantine after possible exposure to COVID-19, which the governor's office announced Sunday. A member of Beshear's security detail had tested positive on Saturday, according to the governor's office.

Up to half of COVID-19 deaths in the United States could have been prevented, according to a research letter published in JAMA Monday.

The research relies on modeling, meaning it is an estimation by scientists that's subject to change as we discover more about the virus and not an official count of preventable deaths.

The researchers sited weak public health infrastructure and the United States' "decentralized, inconsistent" response to the pandemic as key factors driving preventable deaths and noted that the U.S. population is on the younger side but includes a disproportionate number of citizens with comorbidities.