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Symbiotic Relationships and Disease



There is a broad range of relationships among microorganisms (microbes) and the organisms that host them. This relationship is dynamic and can be mutually beneficial or a relationship in which the microbe benefits at the host's expense. In mutualisms both members of the host-microorganism relationship benefit from their interactions. In commensalisms the microbe benefits without helping or harming its host. In parasitism one organism benefits and harms the other. Microbes that invade and cause harm to their hosts are pathogens and utilize many adaptations to colonize and proliferate in their hosts. Diseases resulting from pathogens are categorized based on their modes of transmission from host to host and the severity or duration of harm experienced by the host. There is a pattern in the progression of all pathogenic disease states: from the incubation period, when the pathogen begins replicating; to the prodromal period, when symptoms begin to appear; to illness, decline, and ultimately convalescence, when symptoms clear and the host recovers.

At A Glance

  • An association between two or more organisms is a symbiosis. There are five types of symbiosis: mutualism, synergism, commensalism, parasitism, and antagonism.
  • Symbiotic relationships in which none of the participating organisms are harmed can be mutually beneficial (mutualism), benefit only one species (commensalism), or result in a novel effect (synergism).
  • Symbiotic relationships in which a participating organism is harmed or inhibited can be parasitic or antagonistic.
  • The pathogenicity, or the capacity to cause disease, of an organism depends on its ability to invade, multiply, and avoid host defenses.
  • Virulence, or the severity of a disease caused by a pathogen, is enhanced by structural, molecular, or physiological factors that increase a microbe's ability to invade its host.
  • Diseases are classified as infectious when caused by infectious agents; they are considered communicable if they spread easily from host to host and are further categorized based on their intensity and longevity.
  • Communicable disease can be spread vertically from mother to fetus; directly through physical and sexual contact; indirectly through food or air; via animal vectors that carry pathogens to hosts; and through contamination, which is the presence of a pathogen in nonliving matter, such as food or soil.
  • Diseases are characterized by duration and intensity. Acute illness is rapid in onset, lasts for a short period of time, and responds well to treatment. Chronic diseases develop slowly and are prolonged over years, and many have no cure. Latent disease is not clinically active, but the pathogen remains dormant inside the host body.
  • Diseases go through five stages: incubation period, prodromal period, period of illness, period of decline, and period of convalescence.