A close friend of my family nearly died of what we presume was Covid-19 in February of 2020. All the right symptoms, but nobody had enough tests at the time. By the time they gave her a Covid-19 test, she was negative for Covid, but severely ill from the damage to her lungs.

She presumed she couldn't get it again. She did in May of 2021. Less serious, but tested positive this time with mild symptoms.

Perhaps the first infection wasn't covid-19, but something very similar. My guess is that immunity after infection doesn't last as long as we might all like. Which is why we all need to get vaccinated as quickly as possible.