Virulence is the ability of an microorganism (pathogen) to cause damage to its host. This is done by microorganisms releasing virulence factors which are molecules produces by bacteria, fungi or virusses. These virulence factors can bind to host cells, causing damage to the immune system of the host.
Gut microbiome plays an important role in protection of the host from enteric bacterial infection. However, many pathogens have developed strategies in order to be able to outcompete the host intestinal community, leading to infection and/or chronic diseases.