Our bodies are designed to keep out infection, using an armoury that includes a tough outer layer of skin, tiny hair-like structures in the respiratory tract that propel microbes away from the lungs and gut flora that repel unpleasant bugs. The micro-organisms attempting to penetrate these fortifications include bacteria, viruses, fungi – such as candida – and protozoa, like the insect-borne Plasmodium which causes malaria. Prions – rogue proteins that enter brain cells and turn normal proteins into replicas of themselves – are thought to be behind illnesses affecting the brain. Until the discovery of prions, it was thought impossible for a protein to cause infection.
The more we learn about infective agents, the clearer it is how little we know. Stomach ulcers, it transpires, are not caused by spicy food, stomach acid or stress, but by the bacterium helicobacter pylori (a discovery for which two doctors were awarded the 2005 Nobel Prize for Medicine). New research has shown a link between bacteria in the gums and an increased risk of heart attacks and stroke.
“Every second of the day there is a battle going on within the human body between the micro-organisms within us and those from outside,” says Professor Dilip Nathwani, honorary professor of infection at the University of Dundee. “Whether infection occurs depends partly on the virulence of the organism and the individual’s ability to mount an immune response. One person with a very virulent microbe may experience minimal disease and another whose ability to fight infection is compromised can be made very ill by a low-grade pathogen.”
Serious immune deficiencies such as those caused by gene defects or by infections like HIV are well-researched, says Dr William Egner, consultant immunologist at Sheffield Teaching Hospitals. “But individual minor variations in the immune system are less well understood.” Genes undoubtedly play a part: one person in 500 does not make the IgA antibody that helps keep respiratory infections at bay, while in children a specific protein deficiency can increase susceptibility to ear infections.
However, vulnerability to infections like thrush, cold sores, or colds is not always due to a weakened immune system. “People ask why they get more colds than their neighbour and why they cannot fight them off,” says Dr Egner. “There may be many complex reasons and in most cases, the role of the immune system has not been established. People attribute a wide variety of symptoms to a poorly functioning immune system when in many cases there is no evidence that it is involved at all.”
Dr Egner is also uncertain about the effect on our immune system of diet and lifestyle. “Being malnourished clearly affects the immune system and make people more susceptible to infection but there is no good evidence for the widespread belief that people are more prone to infection when they feel run down.”
Feeling chilled, however, can make you more likely to pick up a cold, as our mothers always insisted. Research from Cardiff University’s Common Cold Centre showed that 29 per cent of people who sat with their bare feet in a bowl of icy water for 20 minutes contracted a cold within the following few days, compared with nine per cent of the control group.
The battle against serious infection is waged through improved social conditions, scrupulous hygiene, modifying behaviour that spreads disease, such as unprotected sex – immunisation and medication.
We may win the battle, but we cannot win the war, as antibiotics – once heralded as the ultimate weapon – have proved. Antibiotic resistance has led to MRSA and the resurgence of tuberculosis. Clostridium difficile, which normally lives quietly in the bowel, proliferates with unpleasant results when other gut bacteria are destroyed by antibiotics, causing inflammation of the colon and diarrhoea.