Virtually everyone has a theory about colds and flu and, given the glut of information, it may be hard to separate the facts from plain fiction. Yet, there are data that show what makes us vulnerable to these infections, and how to stay well when the weather turns cold.

Susceptibility and stress Catching the flu is not a random event. Bugs may be getting nastier (see box below), and our poorly ventilated, sometimes crowded, environments may encourage the spread of virulent viruses. But if exposure were the only factor, all of us would get sick every time we were exposed.

In truth, many people in a room may be exposed to a cold or flu virus, but only some become infected. Susceptibility, not exposure, is the key and is likely to be influenced by a number of things.

People who have heart complaints, asthma, chronic kidney disease or diabetes, or who are taking medications (for instance, steroids) are more susceptible to colds and flu. Smokers are also more susceptible to upper respiratory tract infection (URTI; Am J Public Health, 1993; 83: 1277-83).

But predicting susceptibility in an otherwise healthy population is less straightforward. Nutritional status and exercise play a role (see boxes, pp 2 and 3) but, recently, a great deal of study has focused on the role of stress.

The ability of stress to alter immune function, and precipitate and aggravate infectious diseases has long been recognised (J Fla Med Assoc, 1993; 80: 409-11). Today, there is strong evidence that the single biggest risk factor that puts otherwise healthy people at risk of catching a cold may be stress.

In one study, after determining their stress levels by questionnaire, 394 subjects were exposed to common rhinoviruses (the virus causing the common cold). Only 27 per cent of the low-stress group showed clinical symptoms compared with 47 per cent of those at the high end of the scale (N Engl J Med, 1991; 325: 606-12).

In another study, 276 subjects, who had been carefully evaluated for their level of involvement in one or more of a dozen types of social ties - for example, with a spouse or partner, sibling, parent, close friend or fellow worker - were given nasal drops containing rhinoviruses.

Susceptibility to colds was reduced as social support increased. Of those with three or less relationships, 62 per cent came down with a cold compared with only 35 per cent in those with six or more types of social support (J Am Med Assoc, 1997; 277: 1940-4).

A repeat of that study (Health Psychol, 1998; 17: 214-23) evaluated stress more precisely (long term vs short term) as well as the severity of illness, and also measured blood levels of natural-killer (NK) cells, the immune system cells that combat infectious agents. Those stressed for more than a month had lower NK-cell activity and were 2.2 times more likely to develop colds. The biggest risk came with increased job stress.

Stressed children have a greater susceptibility to URTI and show measurable decreases in mucosal immunity (J Psychosom Res, 1997; 43: 271-8). Depression is also strongly related to catching a cold (Epidemiology, 2001; 12: 345-9).

Prescription drugs, like antibiotics, can also increase colds/ flu susceptibility. A substantial number of GPs still prescribe antibiotics for colds/flu, even though these drugs don’t work against viruses (J Am Med Assoc, 1997; 278: 901-4; J Am Med Assoc, 1998; 279: 875-7).