Infection, state or condition in which the body or a part of it is invaded by a pathogenic (disease-causing) microorganism or virus that, under favorable conditions, multiplies and produces effects that are injurious. Localized infection is usually accompanied by inflammation, but inflammation may occur without infection.
The 5 classical symptoms of infection listed by early medical writers are: (1) dolor—pain; (2) calor—heat; (3) rubor—redness; (4) tumor—swelling; (5) functio laesa—disordered function.
Pain is especially prominent when the infection is confined within closed cavities and is in proportion to the virulence and extent of the infection. Redness and swelling are not evident when infection is within some rigid tissue or deep within some cavity; they are more apparent when superficial structures are involved. In fact, “discoloration” would be a better term than “redness,” for the color is more blue, or purple in advanced infections, while tuberculosis infections have long been called “white swellings.” Heat may be evident on the surface, but there may be considerable elevation of body temperature even with small infections. “Disordered function” depends upon the part affected as well as upon the virulence. With almost all acute infections, there is either an absolute or relative increase of polymorphonu-clear leukocytes in the blood.