Outbreak investigations.

Epidemic nosocomial infections are defined as hospital-acquired infections that represent an increase in incidence over expected rates. Epidemic-associated infections usually are clustered temporally or geographically, suggesting that the infections are from a common source or are secondary to increased person-to-person transmission. Epidemics are important, because they account for a substantial percentage of nosocomial infections. Furthermore, if infection control personnel thoroughly investigate outbreaks of nosocomial infections, they may identify new agents, reservoirs, or modes of transmission.