I strongly encourage you to use a word other than "weak" in this sort of context. "Susceptible" will do. Ditto for "strong". "Resistant" might suffice.
Plenty of people died of COVID19 who were by no measure "weak" (other than susceptibility to COVID19) and your language suggests that viruses act upon some sort of generically less well-adapted genotype. Viruses kill people who are more susceptible to the effects of the virus (which may also differ from person to person). Those people may have previously been judged "strong".
"Weak" and "strong" are incredibly poorly chosen words to use in discussing susceptibility to disease.
Plenty of people died of COVID19 who were by no measure "weak" (other than susceptibility to COVID19) and your language suggests that viruses act upon some sort of generically less well-adapted genotype. Viruses kill people who are more susceptible to the effects of the virus (which may also differ from person to person). Those people may have previously been judged "strong".
"Weak" and "strong" are incredibly poorly chosen words to use in discussing susceptibility to disease.