It's not possible even for doctors to give a definitive diagnosis to a skin rash without actually seeing the patient. It's certainly not possible for non-doctors to give useful medical advice about skin rashes for people they do not even know, but there are some general categories of skin rashes that may or may not require medical treatment.
Pityriasis is a condition of a spreading rash that can take several weeks or several months to go away, causing itchiness and then skin discoloration if it's not properly treated. Pityriasis rubra starts as a single red patch that expands into an oval shape 2 to 10 cm (a little under 1" to about 4") across. After the first patch becomes noticeable, there can be dozens of red lesions that look like bug bites. In a week or two they begin to ooze, and then they crust over and turn brown if they are exposed to sunlight. Itching caused by this form of pityriasis can be treated with low-dose topical steroid creams, some of which you can buy over the counter. It's really rare for the condition to cause anything more serious than an annoying itch, although in rare cases there is an immune reaction that can lead to swollen joints and lymph nodes.
Pityriasis alba causes barely-red ovals also 1 to 4 cm across to break out on the face, shoulders, and torso. Sometimes the changes in skin color are so slight that they aren't noticed until the next summer when other skin tans and the spots don't. It's a relatively common condition in children and teens aged 3 to 16. There will be up to 25 large spots that may cause itching and loss of pigmentation of the skin. Before they resolve, they can become itchy. They are also treated with low-dose steroid creams.
Neither of these conditions has a clear cause. They can occur after a cold or sore throat. They can occur after use of certain medications. They can follow eczema or contact dermatitis outbreaks. They can be triggered by viral or fungal infections, but they aren't viral or fungal infections. Treating an infection you don't have can make other problems worse. Stick to managing symptoms, and then avoiding permanent changes in skin color. Pityriasis rubra can leave permanent brown spots in its wake if you don't use sunscreeen on the affected areas when they are exposed to sunlight. That's because your skin fights inflammation with pigments, and the combination of sunburn and pityriasis triggers release of a lot of pigment. Permanent lightening of the skin in pityriasis alba can be managed with a combination of psoralen and UV lamp exposure, but it's something you need a doctor's help to get right.
Tinea is a fungal infection of the skin. It most often breaks out on the parts of your body that are most conducive to growing a fungus, your feet, inside your shoes, and your groin, inside your underwear. It almost never breaks out on the same parts of your body as where you will have pityriasis. It can cause itching and oozing, but it won't cause changes in skin color after it heals. Tinea can be treated with antifungals or with low-dose steroids if the itching is severe, but it won't spread like pityriasis.
Lichen planus is a red, itchy, and sometimes painful skin condition that usually starts in the folds at your elbows and knees and spreads from there. It can also spread to your mouth and nasal passages. There's not a lot to be done to cure it, although it also responds to low-dose steroid treatment.
Syphilis causes painless chancres that look a lot like pityriasis, but on the genitals.
It's important not to treat a disease you don't have. When In doubt, ask a doctor.
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