The concept of infection

The body is not sterile. Skin, mouth, gut and airways carry a resident community — the normal microbiota — that is established within days of birth and lives in a stable relationship with the host. These residents are not passive passengers. They occupy attachment sites and consume nutrients, so an arriving pathogen finds nowhere to bind and little to eat. They produce acids, fatty acids and antimicrobial substances, and they keep the immune system alert. This protective effect is called colonisation resistance. The exchange is usually mutual: the host supplies warmth, moisture and food, and the microbes help digest food, produce vitamin K and some B vitamins, and crowd out invaders. The balance breaks in predictable ways. Broad-spectrum antibiotics kill residents along with the target organism, leaving an empty niche that Clostridioides difficile, a bacterium that forms hardy spores, can occupy and cause severe diarrhoea. A urinary catheter carries skin organisms along its surface into the bladder, where they form a biofilm — a slime-enclosed community on the plastic that antibiotics penetrate poorly. This is why the strongest nursing measure against catheter infection is not a drug but asking whether the catheter is still needed and removing it as soon as it is not.

The four types of pathogen differ in ways that decide treatment. Viruses are genetic material in a protein coat with no metabolism of their own, so they must insert their genes into a host cell and hijack its machinery. That is why antibiotics, which attack bacterial structures, have no effect on viral illness, and why antiviral drugs are relatively few and often must be started early. Bacteria are complete living cells. They reproduce by binary fission — one cell divides into two, so numbers can double in as little as twenty minutes under favourable conditions — and they possess structures a virus lacks, such as a peptidoglycan cell wall that penicillin-like antibiotics destroy. Fungi are eukaryotes like us, with a chitin wall and a membrane containing ergosterol; because their cells resemble ours, antifungal drugs must exploit the differences and are often slower and more toxic. Parasites range from single-celled protozoa, such as the organism that causes malaria, to multicellular worms, and each needs its own drug and often its own control programme. Fever has a mechanism as well. Pathogen molecules and immune signals called pyrogens act on the hypothalamus, the brain’s thermostat, prompting it to release prostaglandins and raise its set point. The patient then feels cold and shivers, and the muscles generate heat until the body reaches the new, higher target.

Fever is generally defined as a body temperature of 38 °C or above, and it follows a recognisable sequence. In the rising phase the set point has been raised, so the patient feels cold, shivers and pulls up blankets while the skin is pale and cool. At the plateau, heat production and heat loss balance, and the patient feels hot and flushed. In the falling phase the set point drops, and sweating and vasodilatation release the excess heat. Each phase carries a consequence. Shivering raises oxygen demand and can exhaust a frail patient or strain a damaged heart, and sudden defervescence produces heavy sweating and dehydration. The defensive value of fever lies in several effects: higher temperatures increase the activity of neutrophils and macrophages, speed the multiplication of certain white blood cells, and strengthen the action of interferon, while many pathogens grow best at normal body temperature and reproduce poorly when it rises. For these reasons a mild fever is usually watched rather than attacked. Antipyretics are given mainly for comfort, for a patient in pain, or when fever itself is dangerous — very young infants, people with heart disease or a history of febrile convulsions. The nurse’s priorities are to find and treat the cause, measure and record the pattern, maintain fluid intake, remove excess covers, and never leave a feverish patient without reassessment. A fever is a sign, but it is also part of the defence; suppressing the sign while ignoring the cause helps no one.