The bacterial species have co-evolved with the human race and account for more than 3,000 distinct species of which most are beneficial to the humans. They are a few micrometres in length and outnumber the eukaryotic cells by a ratio of 10,000:1. They have evolved their own strategies for countering the defences of the human host. Pathogenicity of microorganisms is through the expression of the virulence factors, which could be attributed to their genetic or biochemical or structural features, which are responsible for the organism to cause an infection in the host (Oelschlaeger et al. 2002). The dynamic relationship between the host and pathogen is an outcome of the virulence of the pathogen and the resistance or susceptibility of the host to the invading pathogen (Casadevall and Pirofski 2000). © Springer India 2015.