I'm not a doctor, so take this with a grain of salt, but generally something classified as an STI has difficulty surviving outside the human body. It's not impo

I'm not a doctor, so take this with a grain of salt, but generally something classified as an STI has difficulty surviving outside the human body. It's not impossible, and anything that allows that pathogen to get into the bloodstream is a risk factor for the infection to spread. Things like blood transfusions, or unclean practices like needle sharing, could all contribute. Sex is by far the most common means of transmission, but it isn't the only way. It couldn't possibly be; otherwise how would the first human have become infected?