Infectious Diseases result from exposure and contact between a host (human being) and an (uninvited) guest (micro-organism).
Given the fact that billions of micro-organisms are in and around us at any time, overall, infectious diseases are comparatively rare; of the millions of different microbial species, only about 300 are known to cause human diseases.
Besides exposure and contact, factors on the side of the host (genetic background; environment, underlying diseases and their therapy) and on the side of the micro-organisms (pathogenicity / virulence factors) are necessary to result in an infectious disease.
“Colonization” means that a micro-organism can attach on skin or mucous membrane for some time or even indefinitely but does not invade into host tissue and does not cause any symptoms. Colonizers may even induce an immune response.
“Infection” is defined as a micro-organism invading through skin or mucous membranes in to the tissue of a host, leading to no disease (“asymptomatic infection”); or symptomatic disease. It is followed by health, disability or death. Following the infection, microorganisms may persist in the body for a long time and even for life without causing any symptoms, which is called “latent infection”.
Infectious diseases may not only be due to pathogenicity factors of a micro-organism, but may also result from (i.) direct destruction of host tissues (e.g. from viral replication); (ii.) the acute host (immune-) response; and from late immune responses resulting in immune-mediated “post-infectious diseases”. Some infections may cause an immune response that is directed against host-tissue, resulting in an “autoimmune-disease”.
Given the increasing number of microbes, the increasing number of exposures and the increasing number and fraction of susceptible/predisposed humans, it is obvious that infectious diseases will increase in the future. Vaccines and vaccination may help solve this problem.