infection

infection The normal human body is covered with billions of harmless microorganisms: indeed, we each carry more bacteria than the total human population of the world! These, together with the skin and the immune system, serve to protect the body from invasion by harmful, or ‘pathogenic’ microorganisms.

If there is a breach in one of these lines of defence, these pathogens can gain access to the body. Entry may be, for example, via a skin wound, inhalation, ingestion, or sexual intercourse, and may be facilitated by immune deficiency or loss of the normal organisms living on the body, for instance after a course of antibiotics.

As soon as the immune system detects the presence of a pathogen it mounts a response to kill it, which is highly successful in most cases in healthy people. On the rare occasions where it fails, or in people with poorly functioning immune systems, the organism may succeed in establishing itself and cause disease: an infection occurs. The term ‘infection’ therefore encompasses not only the classical ‘infectious diseases’, but also such diseases as boils, thrush, urinary tract infection, and surgical wound infections.

The immune response produces a syndrome of inflammation at the site of the infection. This is characterized by redness, warmth, pain, and swelling, caused by extra blood supply to the area bringing white blood cells to fight the infection. Pus may be formed (a mixture of white cells, dead tissue, and organisms). Usually this stops the infection from spreading. However, if the organisms gain entry to the bloodstream, sepsis or ‘blood poisoning’ may ensue. In sepsis the body's white cells respond by producing vast amounts of chemicals which, as well as helping to kill the marauders, result in fever, flushing, shivering, low blood pressure, rapid heart rate, and, in severe cases, delirium. Sometimes this immune response is more harmful than the infection itself. Conversely, sepsis may be difficult to recognize in patients with suppressed immune systems who cannot mount such a florid response. Finally, some microorganisms are not easily recognized by the immune system at all, so that infection may have few if any symptoms until later in the course of the disease when damage to the body by the organism is well advanced. Examples are the human immunodeficiency virus which causes AIDS, and the prion causing Creutzfeld-Jacob disease.

Hospital infection and antisepsis

For many hundreds of years, fevers and infections were believed to be caused by ‘miasmas’, or noxious air exuding from rotten materials. In the nineteenth century the most notorious, and perhaps the most tragic, manifestation of sepsis was puerperal sepsis, or childbed fever, in which the dangerous bacterium Streptococcus pyogenes (now known as the Group A streptococcus) gained entry to the bloodstream via the birth canal. It had a very high fatality rate and was responsible for the deaths of countless young mothers every year. Although well-recognized as a complication of childbirth, the cause was not understood. The Hungarian obstetrician Ignaz Semmelweis, working in Vienna in the 1850s, was particularly concerned by the high rate of childbed fever on one of his wards which was attended by medical students. On this ward nearly a fifth of his patients died of sepsis. On his other ward, attended only by midwives, the rate was only about 3%. He realized that the medical students came directly from the autopsy room to the obstetric ward and proceeded to examine the patients without even washing their hands in between. He insisted that each student should wash his hands with soap and water and then an antiseptic before entering the ward, and saw the mortality rate drop immediately to less than 2%. Thus he proved not only transmission by hand of an infectious agent, but also that it could be prevented by use of antisepsis.

This was a dramatic result, but despite this Semmelweis was ignored and even ridiculed. It was Joseph Lister, working in Glasgow in the late 1860s, who brought about the general acceptance of surgical antisepsis. He used carbolic acid to transform surgery from a highly dangerous last resort to the treatment of choice in many conditions. Florence Nightingale did the same for hospitals after the Crimean War, during which she had shown that cleanliness and hygiene were paramount in preventing injured soldiers from dying of infections — although, ironically, she never believed in the germ theory of disease, rather she believed that filth and dirt bred disease directly.

Since then the refinement of antisepsis before and during operations has been one of the most important developments in allowing the practice of surgery as we now know it. Even now, maintaining a low infection rate is one of the priorities of every surgeon. Low levels are attained by the use of ‘asepsis’ — that is, sterilizing the instruments so that no microorganisms are present on them — and ‘antisepsis’ — the use of chemical solutions to decrease the number of the patient's and the surgeon's own microorganisms as far as possible. Nowadays, one of the greatest challenges facing hospital infection control is the prevention of spread of bacteria that are resistant to many antibiotics, such as methicillin-resistant Straphylococcus aureus (MRSA).

Angharad Puw Davies

See also infectious diseases; microorganisms.