The good news today is that it appears people who get COVID-19 develop antibodies. This may protect them from getting it during future outbreaks. Neither age nor the degree of symptoms seems to make a difference. Everyone who contracts it, develops antibodies. This may lead to a vaccine to lessen the chance of becoming infected with the virus and maybe reduce the symptoms.
If I were in SF 1900's situation. I would get a third test even if I developed symptoms of the virus.