Streptococcus suis is a zoonotic pathogen that generally causes disease in animals, most notably pigs. However, increasing numbers of human cases have been reported worldwide, suggesting that this organism is potentially an emerging cause of human disease. The bacterium is a gram-positive alpha-hemolytic Streptococcus related to Lancefield group D. Human cases of this disease are relatively uncommon; more than 700 cases worldwide have been reported to date. Affected individuals usually have had close contact with infected pigs or products derived from infected pigs. Those at risk include pig farmers, hunters, abattoir and meat-processing workers, and veterinarians. In the history of this organism's epidemiology, there have been two large-scale outbreaks in China (1998 and 2005); however, most cases are sporadic. While most reported human cases have had exposure to pigs, ruminants, cats, dogs, and horses can spread the organism as well.

The most common clinical presentations of this disease include meningitis and sepsis with multi-organ system dysfunction, which carry a high degree of case fatality. Other reported clinical manifestations include peritonitis, endocarditis, cellulitis, arthritis, pneumonia, empyema, and a toxic shock-like syndrome. There are 35 serotypes, but most S. suis infections in humans are associated with serotype 2 strains.

Cases are usually in adult males (rarely ever in children), with a mean age of 47-55 years and a male-to-female ratio of 3.5:1.0. By specific exposure, those involved in slaughtering carry the highest risk (odds ratio 11.9 in a matched case-control study). Pediatric cases have been described in very few case reports. In both immunocompetent and immunocompromised hosts, transmission occurs through occupational exposure.