Host-microbe interactions

Microorganisms are found everywhere and encounters between humans and microbes are frequent. Additionally, different parts of the human body are colonized by bacteria. These bacteria may be harmless or beneficial (commensals), or disease-causing (pathogenic). Pathogens can be transmitted directly or indirectly (through ingesting contaminated food or water or touching contaminated surfaces). Humans have various defenses against the transmission of microorganisms, such as the skin or mucous membranes as physical barriers, the flow of saliva or mucus to carry away microbes or low pH or bile salts, which damage microorganisms. Commensal bacteria (also known as the normal flora, microbiota or collectively as the microbiome) can also help protect against pathogens by their inhibiting the growth or other microbes or the competitive advantage they have in the particular niche. Finally, the immune system allows the host to initiate a specific protective response to neutralize a microbial invasion. When a pathogen breaks the host defenses it may cause infectious disease.