Tomatoes are sour, slightly sweet and moderate in nature. Tomatoes are quite acidic and cooked tomatoes can help diarrhoea, removing too much heat in the body (ie nightsweats). If eaten raw they can help constipation caused by fire in the large intestine.
Tomatoes are rich in vitamin C and stimulate and strengthen the immune system to prevent flu and colds. However because of their acidic nature, tomatoes can cause problems with allergies and over-active immune system problems like diabetes, lupus, cancer etc.
Tomatoes, like most foodstuffs should be eaten in moderation, ie once or twice a week and not everyday in salads or sauces. Tomatoes should be cooked moderately or eaten raw. Overcooking will cause excess acid and damage the nutrition.