Microbial infection is the combinatorial process of invasion of infectious agents into the organism, their multiplication and the reaction of host tissue against these agents. Infectious agents mainly include bacteria, virus, parasite, fungi etc.Primary Infections: Primary skin infections have a characteristic clinical picture and disease course, are caused by a single pathogen, and usually affect normal skin. Impetigo, folliculitis, and boils are common types. The most common primary skin pathogens are S aureus, β-hemolytic streptococci, and coryneform bacteria. These organisms usually enter through a break in the skin such as an insect bite. Many systemic infections involve skin symptoms caused either by the pathogen or by toxins; examples are measles, varicella, gonococcemia, and staphylococcal scalded skin syndrome. Dermatophytic fungi have a strong affinity for keratin and therefore invade keratinized tissue of the nails, hair, and skin.