Have you ever had an itch that just won't go away? One that screams out to be scratched, night and day, and gets worse the more you touch it? One that makes you dig your nails into your flesh until it bleeds, but still demands you scratch it? If you haven't, count yourself lucky. Itchy rashes, whether caused by allergies, eczema, infections, dermatitis or parasites, are one of the most common, poorly understood and difficult to treat conditions people suffer. But recently, researchers have come closer to finding out what causes the itch. Cytokines are proteins involved in making cells do the things they do - including dividing, growing and surviving. These proteins are particularly important for setting off immune system reactions, and are factors in a wide range of diseases. So far, more than 60 of these interesting proteins have been discovered, and recently a new one, called interleukin 31, has been found that's thought to be a key player in causing the itchy rashes in allergies, dermatitis, psoriasis and eczema. If the protein is crucial in causing the itch, it should be possible to find ways to stop it from driving sufferers crazy. This particular cytokine is produced by T-cells - the immune cells needed to attack viruses and other invaders trying to get through our skin. So, finding a way to tell these cells not to produce it at the wrong time is another possibility. But before you can benefit from these or any other skin treatments, you need to know what kind of rash you have. Treatments vary greatly and what's good for one problem may be a disaster for another. Getting your itch diagnosed correctly is the first and most important step. Here are a few clues. If you have a fever when your rash appears, the chances are it's caused by a virus. You need to get that virus diagnosed right away because it may be highly infectious and could progress to a more serious illness. If the rash is made up of raised, itchy lumps that start in one part of the body - often the chest - then spread everywhere, it may be an allergic rash. You need to think about what you may have eaten, any medication you may be taking, or whether you've started using a new kind of make-up, shampoo or washing powder. There are treatments to control this kind of itch, with an antihistamine injection or tablets often settling it. But it's equally important to find out what caused the allergy, because the next time you're exposed to it the reaction could be worse. Dermatologists and allergy specialists can do patch testing to find what caused your reaction. Once you know what caused it, you can take measures to avoid the trigger for the rest of your life. Often it's something you've had no problems with before - many people develop allergies for no clear reason. Other causes of itchy rashes that get specific treatment are fungal infections or parasites. The cleanest of us can end up with an itchy rash thanks to body lice picked up when travelling. Don't be horrified, just get it treated. Fungal rashes can be diagnosed by scraping the itchy lumps and checking for fungus under the microscope. The rest of the itches - and this is a lot of itching - belong in the dermatitis/eczema and 'others' category. It's this big group about which the medical research mentioned above offers most hope. But there are also plenty of other treatments worth trying in the meantime.