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.
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.