The economic, social and personal costs of bacteria, viruses and other small organisms are increasing.

There are some 1415 species of infectious organisms (or loosely speaking "bugs") that are known to be pathogenic to humans including 217 viruses and prions, 538 bacteria and rickettsia, 307 fungi, 66 protozoa and 287 helminths (Taylor et al. 2001).

In recent years new diseases have appeared or old diseases have re-emerged. The geographic distribution of many diseases has broadened due to increased human travel, global trade, habitat destruction and climate change.