I want to share the story of Staphylococcus epidermidis.
This species is a bro. It's all over the skin, like the name suggests. It prevents colonization by other organisms, helps maintain healthy skin chemistry, and even trains the human immune response. Like almost all microbes that have some health effect, most strains are neutral (or beneficial) while a tiny minority has the potential to cause problems. In fact, it's so benign that people with central venous catheters or PICC lines frequently have asymptomatic bacteremia -- a silent Staph epi population in the blood that's not doing a darn bit of harm.
Still, it's the second leading cause of injection site infections. Why?
There's a fuckton of it, and it's everywhere. Ten thousand to ten million cells per square cm, all over your skin. If you wash and swab with alcohol or iodine, you drop the concentration but it's never zero. All of us here have introduced this into organisms IM or subq a gang of times, but the statistics are on our side. They're on our side unless you routinely make bad bets, that is.
The upshot: you cannot avoid introducing bacteria to yourself, you can only stack the deck in your favor. Wash, swab, keep needles covered before use, use clean ones, etc. This is devastatingly easy to do and being this fucking lazy can kill you.
This isn't even harm reduction 101, it's that class they make kids take the summer before 101 because they get word from the last teacher that you drool a lot.