As you said, disease agents can get into our bodies by any open connection that we have already as our mouth, eye and nose, and also by any open entrance created by cuts, open skin,etc.
Because of this we need to avoid bad habits as grabbing our eyes or scratching them, etc.
#1 Tip: Washing hands before eating, after being on the streets, after playing with an animal, etc. This is one of the cheapest ways of preventing disease cause by agents.
God Bless Us All!
RE: How Disease Agents Enter Our Body