Introduction
Athletes engaged in heavy training programmes, particularly those involved in endurance events, appear to be more vulnerable than normal to infection. Sore throats and flu-like symptoms are more common in a thletes than in the general population. There is some evidence that this increased susceptibility to infection is due to a depression of immune system function in these individuals.

There are two main factors that contribute to making an athlete infected: one is the actual exposure to pathogens, foreign microbes, bacteria and the like that can cause disease effects in the body; the other is the impact of different forms of stress on immune function, the immune system being principally the white blood cells and the lymphoid tissues of the body that help to protect the body against these potentially invading micro-organisms that then produce infections. In general it's now recognised that various forms of stress can result in impaired immune function. These can include the physiological stress of heavy exercise, psychological stress (worry and anxiety), exercising in an extreme environment (heat, cold, altitude) and inadequate diet, particularly inadequate intake of either carbohydrate, protein or micro-nutrients.

If we didn't get exposed to these micro-organisms it wouldn't be a problem, but we live in contact with other people, and we breathe in what they breathe out. This exposes us to pathogens, and with a depressed immune system we're more likely to get infected. Among athletes the most common form of infection is the sore throat, infections of the upper respiratory tract.

In general the currently accepted model of upper respiratory tract infection - URTI - in relation to training load is the J-shaped model (fig 1). With moderate exercise you might get some small improvement in your immune system - there is some evidence, mostly in elderly, sedentary people. Most of the evidence at the moment has been collected on long-distance runners. After the Los Angeles marathon 13% reported symptoms in the week afterward; of the control group - people who'd trained but for some reason didn't run - it was only 2%.

Leukocytes, white blood cells, are what normally protect us against invasive infections, and their circulating numbers and functional capacities may be decreased by repeated bouts of intense prolonged exercise. The total mean numbers in trained cyclists are around 30% below those in the sedentary population.

Minimise the chances of developing immunosuppression and/or overtraining Allow sufficient time between training sessions for recovery. Include one or two days resting recovery in the weekly training programme. More training is not always better. It appears that it is the absence of recovery days during periods of heavy training, rather than the intensity of training, that predisposes to development of the Overtraining Syndrome. Avoid extremely long training sessions. Prolonged strenuous exercise (more than 2 hours) is associated with a depression of immune function that can last for several hours post-exercise. Restrict continuous activity to less than 2 hours per session. For example, a 3-hour session might be better performed as two 1-1/2-hour sessions, one in the morning and one in the evening. The same amount of training spread over the course of the day is associated with less immuno-suppression than if the daily training is performed in a single prolonged session. Monitor mood, feelings of fatigue, and muscle soreness of your athletes during training; keep a daily log - the Daily Analyses of Life Demands in Athletes (DALDA) questionnaire is useful for identifying increasing symptoms of stress and failure to cope with the training. Decrease the training load if the normal session feels harder than usual. Discuss with your athlete the importance of regular and adequate sleep (at least 6 hours per night). Sleep deprivation is associated with immuno-suppression, even in sedentary people. Individuals may need more rest after travel across time-zones to allow circadian rhythms to adjust. Diet is important and many vitamins and minerals are associated with the ability to fight infection, particularly Vitamin C, Vitamin A, and zinc. A good well-balanced diet should provide all the necessary vitamins and minerals, but if fresh fruit and vegetables are not readily available, then supplements should be considered. But athletes must be warned not to overdo it: some supplements have immuno-suppressive effects if taken in excessive amounts (e.g. zinc) . Ensure your athletes have an adequate total dietary energy, carbohydrate and protein in-take. Make athletes aware that periods of carbohydrate depletion are associated with immuno-suppression. Low carbohydrate diets worsen the degree of exercise-induced immuno-suppression. Your athletes should be encouraged to consume sports drinks containing carbohydrate before, during and after prolonged workouts. It is important that this is practised during training as well as competition. Drinking carbohydrate 'sports' beverages before, during and after prolonged workouts appears to reduce the adverse effects of exercise on immune function (provided that the exercise is not performed to the point of fatigue). Anything that makes the normal training session feel harder is likely to make exercise-induced immuno-suppression worse or last for longer. Exercising in the heat, becoming dehydrated, exercising at altitude, exercising with sore muscles, during an infection or after inadequate sleep or insufficient carbohydrate intake will increase stress hormone responses, making the depression of immune function worse and increasing the risk of developing the Overtraining Syndrome. Coaches and team doctors might consider discussing anti-flu vaccination with their athletes. Influenza vaccines take 5 - 7 weeks to take effect, and intramuscular vaccines may have a few small side effects, so it is advisable to vaccinate out of season. Individuals with no history of asthma or regular flu will have to pay. Don't vaccinate precompetition, or if an individual has symptoms of illness. Periodisation of training will help to avoid the athlete becoming stale. Training monotony is a risk factor for the development of the Overtraining Syndrome. Training monotony does not mean 'boring training routines', but a lack of short-term day-to-day variation in the training load. A good weekly training strategy is to aim for three days of high or very high training load per week, alternating each day of heavy training with a day of lower training load, and including one day of complete rest every week. An athlete recovering from overtraining should begin with very light training and build up gradually. A common mistake is to resume training at the normal level. They should aim to progress such that they achieve their normal training level after several weeks (i.e. 6 - 12 weeks). Stop, or markedly reduce training if symptoms recur.

Minimise the potential for transmission of infectious agents Athletes should avoid contact with people with symptoms of infection and those who are 'just coming down with a cold'. Infections are passed on by direct or indirect personal contact or inhalation of air close to infected people. Athletes should minimise contact with children of school age and avoid large crowds. Individuals should wash hands regularly, particularly after touching surfaces that are frequently handled by the public, such as doorknobs, handrails and telephone receivers. Encourage your athletes to avoid hand-to-eye and hand-to-mouth contact. This prevents the transfer of microbes to sensitive mucosal tissues. Explain to your athletes the importance of good oral hygiene. Because of the anti-bacterial actions of saliva, sportsmen and women should avoid getting a dry mouth, both during competition and at rest; this can be done by drinking at regular intervals and maintaining hydration status. Various components of saliva offer protection against microbes that cause upper respiratory tract infections. Saliva production falls during exercise, but this can be prevent by regular fluid intake. Athletes should never share bottles or cutlery. Ensure athletes use properly treated water for consumption and swimming. Athletes should avoid shared saunas, showers and jacuzzis. Make athletes aware that they may be particularly vulnerable after training or competition. In terms of infection risk, this is not the best time for them to socialise. Explain to your athletes that good personal hygiene and thoughtfulness are the best defences against respiratory infection.

Training and infection Exercise tolerance may be reduced when athletes have an infection. Exercising with an infection may increase the severity and/or duration of the illness, although light exercise during convalescence may enhance recovery. Training should be stopped if an individual has a fever. The athlete's body temperature will already be higher than normal. Exercising during a fever will result in premature fatigue and the danger of heat stroke. Athletes should not resume training at the same level; build up gradually. Team members with infection should be isolated as much as possible from the rest of the team.

The first line of defence against pathogens that come in through the nose and mouth is the fluid lining the mucosal surfaces, which contains antibodies. If you exercise above a light intensity you're going to start breathing through your mouth, and breathing deeply. A dry mouth increases the athlete's vulnerability to infection. Drinking regularly throughout the exercise, especially a flavoured drink, will stimulate saliva production and improve your defence.

Athletes will be most vulnerable in the first few hours after a training session or competition. After that most things start to recover, though some people take longer than others.

One possible cause of the diminution of immune function associated with heavy training may be that the cumulative effects of repeated bouts of intense exercise, with the consequent elevation of stress hormones (particularly glucocorticoids), may cause temporary immunosuppression. When exercise is repeated, in many cases there may not be sufficient time for the immune sytem to recover fully.

Given that heavy training appears to be linked with impaired immune function, athletes and coaches should employ training strategies to minimise the risk. For most athletes and coaches balancing the fine line between hard training and overtraining is one of the most difficult (yet most fundamental) parts of obtaining an appropriate training response. Most training programmes require the athlete to overload (or overreach), in which a relatively brief decrease in performance may be experienced, but which is followed by a full recovery or improvement in performance (adaptation or supercompensation). This is in contrast to the Overtraining Syndrome in which the athlete experiences long-term underperformance and fatigue, despite continued or even increased training. Unfortunately, it is difficult to distinguish between overloading and the early stages of overtraining. Commonly-reported symptoms of overtraining include: muscle soreness and weakness, reduced exercise tolerance, reduced motivation, mood swings, sleep disturbance, recurrent infection, loss of appetite, and diarrhoea.

Although there are many symptoms of overtraining there are few, if any, reliable markers of impending overtraining. An increased resting heart rate after waking up in the morning may indicate overtraining, but a more sensitive and reliable measure is the heart rate measured by radio telemetry ('sports tester watches') during sleep. In addition, a number of measures can be made on blood samples that are potential markers of overtraining. Clearly this requires the help of an accredited exercise physiologist, cannot be performed on a frequent (i.e. weekly basis) and may be expensive. Finally, psychological profiling may be a useful tool for identifying impending overtraining. Simple questionnaires can be used daily to highlight mood disturbances and symptoms of stress which often appear during the early stages of overtraining. An example of such a questionnaire is the Daily Analyses of Life Demands in Athletes (DALDA).

If the athlete and coach fail to notice the early signs of overtraining, and the athlete is unfortunate enough to develop the overtraining syndrome, it is usually advisable to dramatically reduce or stop training for 2 - 3 weeks. Rehabilitation of overtrained athletes often requires considerable effort to hold them back. A common mistake is to resume training at the normal level. One should begin with light training and build gradually with careful monitoring. The process may take many weeks (i.e. 6 - 12 weeks).

Nutrient availability has the potential to affect almost all aspects of the immune system, because many nutrients are involved in energy metabolism and protein synthesis. Athletes are generally advised to eat a well-balanced diet made up of a variety of foods in sufficient quantity to cover their energy expenditures. But diets high in protein or CHO or fat, very low energy diets, megadoses of vitamins and minerals are commonly used. These dietary extremes may compromise immune function. Diets that are very high in CHO, for example, are generally low in meat products and so may lack sufficient protein and vitamin B12, which is essential for DNA and RNA synthesis. Excessive amounts of specific nutrients can also have detrimental effects.

Insufficient CHO during exercise over several days is associated with elevated cortisol levels. The results of various studies into the effects of CHO on saliva production suggest that CHO intake before and after exercise may help to offset some of the detrimental effects of exercise on immune function. A recent study that investigated the effects of endurance training for 7 weeks on CHO-rich (65% of dietary energy) or fat-rich (62% of dietary energy) diets concluded that diet during training may influence natural immunity since NK cell activity increased on the CHO-rich diet, compared with the fat-rich diet, in response to training.

Athletes require approximately double the intake of protein of the sedentary population. It is well-accepted that a prolonged inadequate intake of protein impairs host immunity with particularly detrimental effects on the T-cell system, resulting in increased incidence of opportunistic infections. However, provided that athletes consume a well-balanced diet that meets their requirement for energy, the increased requirement for protein will be met. Those most at risk would be those restricting food intake in order to lose weight, vegetarians, and those consuming unbalanced diets, There is no indication in the literature to suggest that vitamin intake is insufficient among athletes in general - they tend to ingest above average quantitities, and it may be that, as with dietary protein requirements, any increase in need is countered by increased dietary intake. However, vitamins with anti-oxidant properties (including vitamins C, E and B-carotene) may be required in athletes in order to inactivate the products of exercise-induced increased free radical formation.

As training advances athletes tend to develop dose related mood disturbances with low scores for vigour, and rising scores for negative moods such as depression, tension, anger, fatigue and confusion. These mood changes may reflect underlying biochemical or immunological changes that are communicated to the brain via hormones and cytokines. Stress is a non-specific response to any demand, physical, physiological, or psychological, and it is likely that in many situations, these effects are additive. Extreme stress can result in breakdown.

Traumatic life events are perceived as stressful and generally result in depressed immune function and increased incidence of infection. For example, subjects who were assigned to a high-stress group on the basis of their responses to a life-events history questionnaire showed more frequent URTI than a low-stress group. Furthermore, in the high-stress group the duration of each episode was longer than for the low-stress group.

Elite athletes have to train hard to compete successfully, so some degree of physical training stress is unavoidable. In addition there is the psychological stress of competition, team and commercial pressures, international travel, selection pressures, funding pressures, and other major life events. The aim of the coach, working with a sport psychologist, should be to anticipate these additional stressors and, through appropriate evaluation and planning, eliminate or minimise as far as possible their impact upon the athlete. Appropriate strategies may include realistic evaluation, thorough performance preparation, imagery use, the development of the athlete's own coping skills, and access to social support.

Competing in international high profile events, especially in a foreign country, imposes many psychological stresses on elite athletes. Allowing contact with family members and friends (if only through phone calls), provision of familiar music, videos and food may help to distract from and minimise the impact of stress. During training psychological profiling may be undertaken to some effect, using selfscored profiles of mood states (POMS). Some scientists believe that the best gauge of excessive training stress is how the athlete feels.

Athletes are often required to compete in extremes of heat or cold. For many endurance athletes periods of training at altitude may be required. Exercising in these conditions is associated with an increased stress hormone response and perception of effort. The general consensus is that exahausting physical activity and severe environmental stress generally have at least an additive effect on stress responses, including immuno-suppression.

With frequent international competition now being the norm in many sports, competitors are faced with regular air travel and the associated problems of sleeplessness, jet-lag, and limited food choices. Travelling for many hours in a confined space with several hundred other individuals (a certain proportion of whom are bound to have infections) and rebreathing the same dry air in hypobaric conditions is highly conducive to the spread of infection. Precautions are recommended, including the wearing of a filter mask, maintaining hydration, avoiding alcohol, and trying to get some sleep.

Other behavioural, lifestyle changes, such as good hygiene practice, may limit transmission of contagious illnesses by reducing exposure to common sources of infection, including airborne pathogens and physical contact with infected individuals.