At the minute im not to sure on whats going on, I constantly think there is something wrong with me?

This has been going on for a long time, I could have a headache one minute and think its a Brain tumour or a lump the next and think its cancer. This has been going on for years and i cant cope with it anymore. Sometimes im that scared to leave the house because im terrified of catching some incurable disease. Can anybody help me??