Someone once showed me that a doctor washing his hands was like a religious ritual that would be proven to be like superstition in the next age where everything could be inscribed on The Real.
Organisms are everywhere, inside, outside, etc. You live with them because they are what-I-am even before you can define yourself.
Still, you have to practice risk management in everything. It's about the cost syringe for some people. Others, it's about the convenience in a world that is not convenient for syringe users (they just take one syringe with them and reuse it). Most people reuse daily or even weekly without any sort of issue. A very few get infections, especially subcutaneously.
These are the sorts of decisions we struggle with in the cost of care. So really what's it worth?