Bacteria can be transmitted to and from humans via direct or indirect mechanisms.
Direct transmission can occur through person-to-person contact, when an infected person touches or exchanges bodily fluids with someone else. For example, the bacterial pathogens Chlamydia trachomatis and Neisseria gonorrhoeae are spread by sexual contact. Indirect transmission can occur through other animals (vector-borne transmission); for example, the bacterium that causes plague, Yersinia pestis, is transmitted by fleas. Indirect transmission can also occur through inanimate objects or substances (vehicle-borne transmission).
Examples of vehicle-borne transmission
|Transmission route||Disease (causal bacteria)|
|Respiratory droplets||Tuberculosis (Mycobacterium tuberculosis), Pneumonia (Streptococcus pneumoniae, Haemophilus influenzae and others)|
|Food||Food poisoning (Salmonella enterica, Campylobacter jejunii and others), Shigellosis (Shigella species)|
|Contaminated water||Cholera (Vibrio cholerae), Legionnaires’ disease (Legionella pneumophila)|
The transmissibility of a bacterium in an outbreak determines its basic reproduction number or R0 (R zero). This is the average number of secondary infections that result from each infection. This metric is useful because it influences how an infectious disease will spread through a population - for an epidemic to occur in a susceptible population, the R0 must by >1. Transmissibility depends not only on the mode of transmission, but also on the infectious dose of the bacterium, which is the number of organisms required to establish an infection. For M. tuberculosis, the infectious dose is thought to be very low at fewer than 10 bacteria, whereas thousands of V. cholerae bacteria are required to cause cholera.
An outbreak or epidemic is a sudden increase in the occurrence of an infectious disease in a specific time or place. This could involve a single ward in a hospital, or could involve hundreds of people across a country. The classification of an outbreak is very context dependent, so for very rare pathogens they can involve as little as two cases. A pandemic is an outbreak occurring across multiple continents.
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