An 18-year-old died of COVID-19. It was the second time he contracted the disease, relatives say.

• Wilber Portillo, from Denver, Colorado, died November 19, and days later his COVID-19 test came back positive.
• The 18-year-old tested positive for COVID-19 during the first week of October, but he quarantined for a month then recovered and tested negative, Fox reported.
• He fell ill in November, went to the doctor for a COVID-19 test, and died in his sleep that night, according to his family's GoFundMe.

Wilber Portillo was feeling fine a month after testing positive for COVID-19. The 18-year-old from Denver had quarantined for a month after testing positive for the virus in the first week of October, Fox Denver 31 reported.

Andrea Ferrel, Portillo's girlfriend, told Fox Denver 31 he thought he had developed immunity after he recovered, but within the second week of November Portillo fell ill again.

His condition worsened, and Portillo went to the doctor on November 18 where he was diagnosed with a severe respiratory infection and tested for COVID-19.

Portillo died in his sleep that night. Two days later, his COVID-19 test came back positive, the family said.

His cousin and Ferrel, both told Fox Denver 31 they believe he contracted the virus for a second time after recovering from his first bout of COVID-19.

Scientists are still working to determine whether it is possible to have active COVID-19 for an extended period of time with severe flare-ups. COVID-19 long-haulers, or people who have experienced symptoms of the virus for months, have formed community around their struggles with dealing with COVID-19 long-term.

Tests can also give false-negative results for COVID-19.

Read More: COVID-19 has killed thousands of young Americans. This is not just a tragedy for the elderly.

COVID-19 does not just affect older people

Portillo is among a striking number of young people who have died from COVID-19, despite the common misconception that the virus is generally only dangerous to the elderly.

While the virus is 90 times more likely to kill someone 65 or older than someone 20 to 29, young people are still dying at an alarming rate from the virus, previously reported Business Insider's Aylin Woodward and Susie Neilson.

"Just because you're young you aren't immune to it and even though we see more cases with older people and people with health conditions and even people without health conditions. It's important that you stay home," Ferrel told Fox Denver 31.