Infectious dose (ID) is the amount of pathogen (measured in number of microorganisms) required to cause an infection in the host.
Usually it varies according to the pathogenic agent and the consumer's age and overall health.
Infectious doses for some known microorganisms
The likelihood of infection in all cases is also linked to the immune status of the individual (immunocompromised individuals may become infected more readily).
References
Infectious dose Wikipedia(Text) CC BY-SA