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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

• Salmonella, E. coli : quite large, > 10⁵ of organisms
• Shigella : very low, 10s of organisms
• Cholera : relatively large (10⁴ - 10⁶ of organisms)
• Campylobacter jejuni: relatively low, 500 organisms
• C.parvum : very low, 10 to 30 oocysts
• Entamoeba coli : extremely low, from 1 cyst