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 103 to 108 cells.
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Estimated Infective Dose of Selected Pathogens
Microbes with small infective doses have greater virulence. The presence of a suboptimal dose of disease-causing pathogens does not result in infection.
|Name of the Organism||Primary Route of Infection||Disease||Estimated Infectious Dose|
|Measles virus||Respiratory||Measles||1 virus|
|Noroviruses||Respiratory||Food poisoning||≥18 viral particles|
|Coxiella burnetii||Respiratory||Q Fever||1-10 bacteria|
|Cryptosporidium parvum||Ingestion||Cryptosporidiosis||10-100 oocyst|
|Francisella tularensis||Various||Tularemia||10-50 bacteria|
|Smallpox virus||Respiratory||Smallpox||10-100 viruses|
|Brucella spp||Various||Brucellosis||10-100 bacteria|
|Shigella spp||Ingestion||Shigellosis||10-100 bacteria|
|Various||Mosquito bite||Viral Encephalitis||10-100 viruses|
|Yersinia pestis||Flea bite||Plague||100-500 bacteria|
|Neisseria gonorrhoeae||Sexual contact||Gonorrhea||1,000 bacteria|
|Bacillus anthracis||Respiratory, cutaneous||Anthrax||8,000-50,000 bacteria|
|Salmonella Typhi||Ingestion||Typhoid||10,000 bacteria|
|Vibrio cholerae||Ingestion||Cholera||100,000,000 bacteria|
Example: Infective dose of Bacillary Dysentery vs Cholera
Shigellae are only pathogenic in humans. The pathogens are ingested orally. 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 (≥108), since many vibrios are killed by the hydrochloric acid in gastric juice.
Measurement of Virulence
Virulence is a quantitative measure of pathogenicity related to an organism’s invasiveness and toxigenic potential. Virulence of a pathogen can be measured experimentally by determining the lethal dose 50 (LD50) or the infectious dose 50 (ID50).
- Infectious dose 50 (ID50) refers to the dose or number of organisms that will infect 50% of an experimental group of hosts within a specified time.
- Lethal dose 50 (LD50) refers to the dose or number of organisms that will kill 50% of an experimental group of hosts within a specified time.
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.