A serovar or serotype is a grouping of microorganisms or viruses based on their cell surface antigens. Serovars allow organisms to be classified at the sub-species level; an issue of particular importance in epidemiology.
Serovars may be established based on virulence factors, lipopolysaccharides in Gram-negative bacteria, presence of an exotoxin (pertussis toxin in Bordetella pertussis, for example), plasmids, phages, or other characteristics which differentiate two members of the same species. 
Salmonella, for example, has over 4400 serovars: Salmonella enterica serovar Typhimurium, S. enterica serovar Typhi, and S. enterica serovar Dublin, to name a few.
Vibrio cholerae, which causes cholera, has 139 serotypes, based on cell antigens. Only two of them produce an enterotoxin and are pathogens: 0:1 and 0:139.
Serotypes were discovered by the American microbiologist Rebecca Lancefield in 1933.
- ↑ 1.0 1.1 Baron EJ (1996). Classification. In: Baron's Medical Microbiology (Baron S et al, eds.) (4th ed. ed.). Univ of Texas Medical Branch. (via NCBI Bookshelf) ISBN 0-9631172-1-1.
- ↑ 2.0 2.1 Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed. ed.). McGraw Hill. ISBN 0-8385-8529-9.
- ↑ Lancefield RC (1933). "A serological differentiation of human and other groups of hemolytic streptococci". J Exp Med. 57: 571.