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Pathogens (disease-causing agents) cannot produce an infection which can eventually result to a detectable clinical disease with noticeable signs and symptoms except the infectious agent (i.e. pathogen) meet some certain criteria. The pathogenic microorganism must first gain entry into the susceptible host before it can cause disease. Some of these factors which determine whether or not the infecting pathogen will produce a disease are highlighted. 


  • Infective dose of the microbe: The infective dose of a disease-causing agent is the quantity of an infective agent (i.e. a pathogen) that is required to cause a clinical disease in a susceptible individual. Infective dose can also be called the microbial load of a pathogen in a susceptible human host. Infective dose is the minimal amount of microorganisms that are required to infect a host, and which subsequently cause a clinical disease. It is usually dependent on the mechanism of the molecules that pathogens use to facilitate their infection in the susceptible host.
  • Pathogenicity of the microbe: Pathogenicity is the sole ability of a disease-causing agent to produce disease in a susceptible host. It is the mechanism of disease development in a susceptible individual.
  • Virulence of the microbe: Virulence is the severity of symptoms caused by a pathogen in a susceptible host. Some pathogens have low virulence while the others have high virulence. Microbes with low virulence will produce a less severity of symptoms in a host while those strains that are highly virulent will produce relatively severe symptoms in the same individual.
  • Source of the microbe: The source of a pathogen is the animate or inanimate object from where the susceptible host acquired the pathogen. Normally, the susceptible host comes into contact with the pathogen following a direct or indirect contact with the source (which can be an individual carrying the microbe or a vector or fomites habouring it).
  • Reservoir of the microbe: The reservoir of a disease-causing agent is the natural environment where the pathogen is found; and from where it can go on to infect susceptible hosts in a particular community. Some microbes are found in the soil, in water, in animals and even in humans. A good understanding of both the source and reservoir of a pathogen is vital to the effectual control and management of any disease outbreak.  


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