Viral illnesses are usually simply referred to as a virus or viral infection. They occur when a microscopic infectious agent enters the body. Here the virus attacks the body, causing an immune response and illness.
Viruses can take many forms, some cause a feeling of fatigue for a few days, some cause vomiting and diarrhea or infections (most commonly to the respiratory system) or more serious forms, include ebola, AIDs, cancer and avian influenza (bird flu).
In the case of the athlete, training hard for prolonged periods results in a weakened immune system and increased susceptibility to viruses.
Once a virus has been diagnosed, a decision must be made regarding training and sometimes competition. Those suffering symptoms of vomiting and diarrhea and with a raised temperature are ruled out due to the potential for serious illness developing. Training in the presence of systemic symptoms (such as muscle aching, hot flushes etc) is also contraindicated.
Those with a mild (or no) temperature the athlete may be able to complete a light training session, keeping below 70% of maximum heart rate. This has previously been demonstrated to have positive effects. Competing with a viral illness will in the vast majority of cases result in impaired performance.