Originally Posted by
AirBear
https://news.yahoo.com/covid-19-rein...154615241.html
25 yr old man, originally infected in mid-April. Reinfected in late May with a slightly different strain of COVID-19. 2nd time was much worse. Luckily this is a very rare occurrence and they say usually the 2nd infection isn't nearly as bad as the 1st. This case was a rare exception.
Can happen with almost any virus. As you said, very rare to be re-infected so soon.
Depending on the virus natural immunity may last months, years, or a lifetime. Years is more typical.