Infective dose (ID) and Lethal Dose (LD)

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 OrganismPrimary Route of InfectionDiseaseEstimated Infectious Dose
Measles virusRespiratoryMeasles1 virus
Noroviruses Respiratory Food poisoning≥18 viral particles
Coxiella burnetiiRespiratoryQ Fever1-10 bacteria
Cryptosporidium parvumIngestion Cryptosporidiosis10-100 oocyst
Francisella tularensisVariousTularemia10-50 bacteria
Smallpox virusRespiratorySmallpox10-100 viruses
Brucella sppVariousBrucellosis10-100 bacteria
Shigella sppIngestionShigellosis10-100 bacteria
VariousMosquito biteViral Encephalitis10-100 viruses
Yersinia pestisFlea bitePlague100-500 bacteria
Neisseria gonorrhoeaeSexual contactGonorrhea1,000 bacteria
Bacillus anthracisRespiratory, cutaneousAnthrax8,000-50,000 bacteria
Salmonella TyphiIngestionTyphoid10,000 bacteria
Vibrio choleraeIngestionCholera100,000,000 bacteria

Example: Infective dose of Bacillary Dysentery vs Cholera

Bacillary Dysentery

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 (≥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).

  1. 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.
  2. 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.
Determination of the LD50 of a Pathogenic Microorganism

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.


  1. Chan, T. C., Jiang, J., Temenak, J. J., & Richards, A. L. (2003). Development of a rapid method for determining the infectious dose (ID)50 of Orientia tsutsugamushi in a scrub typhus mouse model for the evaluation of vaccine candidates. Vaccine, 21(31), 4550–4554. 
  2. Aldous, E. W., Seekings, J. M., McNally, A., Nili, H., Fuller, C. M., Irvine, R. M., Alexander, D. J., & Brown, I. H. (2010). Infection dynamics of highly pathogenic avian influenza and virulent avian paramyxovirus type 1 viruses in chickens, turkeys and ducks. Avian pathology : journal of the W.V.P.A, 39(4), 265–273.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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