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By present account 1,407 pathogens can infect humans; pathogens defined here include viruses, bacteria, parasites protazoa and fungi. Infection results in contact between human cells and a pathogen. Chemical interaction occurs between the two and from the standpoint of the human cell can be categorized as non-self recognition, and defense. This local encounter then gives rise to a systemic response to the infection which attempts to favor containment, eradication and elimination of the invader, or it may reflect the toxic effect of the pathogen on the organism as a whole. In some cases the immune response to infection results in late sequelae leading it seems to the development of auto-immune based diseases.