1) keep a detailed journal, what are your symptoms? Does anything precede them be it sun, stress, a new food.
2) If there are visual symptoms such as rashes photograph them. Often by the time you have seen a doctor they are faded or gone.
3) observe the people around you, do you get sick but no one else in the house does? In which case is more likely to be your own body doing something whacky than a viral or bacterial infection.
4) look for patterns in the symptoms you have, if each time you are ill it's the same batch of symptoms again its unlikely to be a virus or other infection. They all tend to have their own MO.
5) Be persistent, no one knows you like you. If you feel consistently ill for no reason you have a right to know why. Don't let people make you feel like a malingerer or pain in the arse.