El Tor is the name given to a particular strain of Vibrio cholerae, the causative agent of cholera. Also known as O1 it has been the dominant strain in the seventh global pandemic. It is distinguished from the classic strain at a genetic level, although both are in the O1 serogroup and both contain Inaba, Ogawa and Hikojima serotypes . It was first identified in 1905 at a camp in El Tor, Egypt .

El Tor was identified again in an outbreak in 1937 but the pandemic did not arise until 1961 in Sulawesi. El Tor spread through Asia (Bangladesh in 1963, India in 1964) and then into the Middle East, Africa and Europe. From North Africa it spread into Italy by 1973. In the late 1970s there were small outbreaks in Japan and in the South Pacific.

More recently there was an outbreak in South America from January 1991 to September 1994, apparently initiated by discharged ballast water. Beginning in Peru there were 1.04 million identified cases and almost 10,000 deaths. The causative agent was a non-O1, nonagglutinable vibrio (NAG) named O139 Bengal. It was first identified in Tamilnadu state, India and for a while displaced O1 in southern Asia before decreasing in prevalence from 1995 to around 10% of all cases. It is considered to be an intermediate between O1 and the classic strain and occurs in a new serogroup. There is evidence as to the emergence of wide-spectrum resistance to drugs such as trimethoprim, sulfamethoxazole and streptomycin.

The extent of the pandemic has been due to the relative mildness (lower expression level) of El Tor, the disease has many more asymptomatic carriers than is usual, outnumbering active cases by up to 50:1. El Tor also remains in the body for longer and survives between than other known types. The actual infection is also relatively mild, or at least rarely fatal. Additionally El Tor is capable of host-to-host transmission, unlike the classic strain.