M.D. Thorids Berger on maintaining good health and immunity:
Prevention is better than cure. This saying, suitable for countless situations in everyday life, also applies to the maintenance of health. It also applies to the fact that prevention of infection is always superior to treatment. Although there is no single method that completely eliminates the risk of contracting an infection, there are several effective ways of reducing the number of infectious episodes incurred over a given period. These means of reducing infection risk include appropriate the management of training loads, the use of appropriate recovery strategies, good personal hygiene, avoiding contact with large crowds and sick people, good nutrition, adequate and good quality sleep and limiting other life stresses to a minimum.
The general consensus on managing training to maintain immune health is to start with a programme of low to moderate volume and intensity; employ a gradual and periodised increase in training volumes and loads; add variety to limit training monotony and stress; avoid excessive long distance training that could lead to exhaustion, illness or injury  ; ensure sufficient rest and recovery; and instigate a testing programme for identifying signs of performance deterioration or physical stress  . Inter-individual variability in recovery, exercise capacity, non-training related stress factors, and stress tolerance should be considered. Variations in performance and fatigue are symptoms to be expected and respected in every athlete.However, these symptoms may indicate periods of increased susceptibility. Athletes and coaches and should be aware of such periods of increased risk and pay particular attention to recovery and nutritional strategies. Fry RW, Morton AR and Keast D. Periodisation of training stress–a review. Can J Sport Sci 17: 234-240, 1992  Gleeson M. Immune function in sport and exercise. J Appl Physiol 103: 693-699, 2007  Pyne DB, Gleeson M, McDonald WA, Clancy RL, Perry C, Jr. and Fricker PA. Training strategies to maintain immunocompetence in athletes. Int J Sports Med 21 Suppl 1: S51-S60, 2000.