Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025
Definition
A primary pathogen is a microorganism that can cause disease in a host regardless of the host's microbiota or immune system status. These pathogens are capable of overcoming the body's natural defenses to establish an infection.
5 Must Know Facts For Your Next Test
Primary pathogens possess virulence factors that enable them to invade and damage host tissues.
They can infect healthy individuals, not just those with weakened immune systems.
Common examples include Mycobacterium tuberculosis and Salmonella spp.
Primary pathogens often have specific mechanisms for evading the host immune response.
They are distinguished from opportunistic pathogens, which only cause disease when the host's defenses are compromised.
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Related terms
Virulence Factor: Molecules produced by pathogens that contribute to the pathogenicity and facilitate invasion and colonization of the host.
Opportunistic Pathogen: Microorganisms that cause disease primarily in hosts with compromised immunity or disrupted microbiota.