In medicine, transmission is the passing of a disease from an infected individual or group to a previously uninfected individual or group.
In order to survive, microorganisms that require human hosts must have a way to be transmitted from one host to another.
Infectious agents are generally specialised for a particular method of transmission.
Taking an example from the respiratory route, from an evolutionary perspective a virus or bacteria that causes its host to develop coughing and sneezing symptoms has a great survival advantage - it is much more likely to be ejected from one host and carried to another.
This is also the reason that many microorganisms cause diarrhea.
Microorganisms vary widely in the length of time that they can survive outside the human body, and so vary in how they are transmitted.