Typhoid is mainly caused by the bacterium Salmonella typhimurium or Salmonella typhi. Other species causing typhoid are Salmonella paratyphi A and Salmonella paratyphi B. They are Gram-negative bacteria. They cannot retain Grams stain in the presence of alcohol. They are rod shaped bacteria measuring 2 - 4 µ in length and 0.5 - 0.8 µ in width. They are motile and possess multiple flagella (Peritrichous flagella). They are facultative anaerobes.