The ability of a parasite to spread from one host to another is known as communicability. This property does not influence the production of disease in an individual host but determines the survival and distribution of a parasite in a community.
In general infection in which the pathogen is shed in secretions, as in respiratory or intestinal diseases, are highly communicable. In come instances, as in hydrophobia, human infection represents a dead end, there being an interruption in the spread of the pathogen to other hosts.
Development of epidemic and pandemic diseases requires the strain of pathogen to possess high degrees of virulence and communicability.
Successful infection require that an adequate number of bacteria should gain entry into the host. The dosage may be estimated as the minimum infecting dose (MID) or minimum lethal dose (MLD) which are, respectively, the minimum number of bacteria required to produce clinical evidence of infection or death, respectively, in a susceptible animal under standard conditions.