I wrote in a previous post about Koch’s postulates. To recap, whether you get an infection or not, depends on 3 things:
1. Host vulnerability – if you are immunocompromised (for example)
2. Virulence of the organism – if you get Ebola as opposed to a mild cold
3. Dose of organism – if you have a huge dose of an innocuous infective organism you are more likely to become ill than if it is only 1 bacterium or virus.