Where should one search when confronted with outbreaks of nosocomial infection?

The outbreak database (http://www.outbreak-database.com), containing 1561 records of nosocomial outbreaks published in the medical literature in a systematic format, was used to identify the most frequent outbreak sources for the entire database as well as for specific outbreak pathogens. Overall, in 37.1%, no source was identified. The main sources identified were index patients (40.3%), followed by equipment and devices (21.1%), environment (19.8%), and personnel (15.8%).