User:Graham Beards/Infection

Pus oozing from an abscess caused by bacteria

Infection is the invasion of the tissues of the body by microorganisms.

The body is continually exposed to numerous microorganisms of many different species, including commensals, which grow on its skin and mucous membranes, and saprophytes, which grow in the soil and elsewhere. The blood and tissue fluids contain nutrients sufficient to sustain the growth of many bacteria. The body has defense mechanisms that enable it to resist microbial invasion of its tissues and give it a natural immunity or innate resistance against many microorganisms.

The innate defense mechanisms are very important to health. These mechanisms are non-specific in that each works against a variety of species. They include mechanical barriers to the entry and spread of microbes, the antibiotic action of the normal commensal flora, microbicidal substances present in the body fluids and secretions, microbicidal substances liberated from tissue cells damaged by the infective process, and phagocytosis by phagocytes. The defenses do not need to be of an aggressive nature, that is, the host might simply fail to supply, for example, a sufficiently high or low oxygen level in a particular organ, or an essential nutrient for a bacterium, or a cell surface receptor required by a particular virus.

Pathogenic microbes are specially adapted and endowed with mechanisms for overcoming the normal body defences, and can invade otherwise sterile sites. Some pathogens invade only the surface epithelium, skin or mucous membrane, but many invade more deeply, spreading through the tissues and disseminating by the lymphatic and blood streams. In some rare cases a pathogenic microbe can infect an entirely healthy individual, but in most cases infection occurs only if the body defense mechanisms are damaged by some locally or underlying debilitating disease, e.g. wounding, intoxication, chilling, fatigue, and malnutrition. In many cases, it is important to differentiate infection and colonization.

Some bacteria that are saprophytic or commensal organisms of the gut, e.g. Escherichia coli, can damage healthy tissues when outside the gut. When an organism succeeds in overcoming the normal body defenses, the infection usually progresses until specific immunity mechanisms become operative, for example, antibodies, or successful antibiotic treatment is given. Acquired resistance to antibiotics by bacteria is becoming a major problem in the control of infection.