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It’s difficult to know the answer to this question as there are so many factors e.g. temperature, humidity, amount and type of bacteria/virus etc...
I would consider cum (semen) to still be infectious while it is moist, STI like HIV, gonorrhea and chlamydia could be passed to you if cum got inside your ass by his fingers.
HIV doesn’t survive well when the body fluid has dried out, but other viruses that pass by sex like hepatitis B can survive for days.
As most STI do not show symptoms we always recommend people go for an STI test if they are concerned something has passed to them.
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This answer was posted on June 21, 2016