The severity and duration of any infectious disease depend on the infective dose of the pathogen and predisposing host factors.
The infective dose is defined as a minimum number of microorganisms required for an infection to proceed. Some pathogens can cause infection only with a small number of cells in the initial inoculum, whereas others require many cells to infect a host successfully. For example, only about ten cells of EHEC (Enterohemorrhagic Escherichia coli) can cause infection. Whereas Vibrio cholerae requires 10 3 to 10 8 cells.
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Microbes with small infective doses have greater virulence. The presence of a suboptimal dose of disease-causing pathogens does not result in infection.
Shigellae are only pathogenic in humans. The ingestion of pathogens are through oral route. Only a few hundred Shigella bacteria are sufficient for an infective dose.
Infection results from oral ingestion of the pathogen. The infective dose must be large (≥10 8 ), since many Vibrios are killed by the hydrochloric acid in gastric juice.
In this example, 30 doses of strain A of a pathogen can kill 50% of host cells, whereas, for strain B, 50 doses are required. As a smaller dose of strain A (compared with strain B) can kill 50% population of host cells, strain A is more virulent than strain B.
