Infective rashes
Candida
This is caused by infection due to, you guessed it, candida albicans which is a skin commensal (i.e. it's there naturally). It causes itchy, red macules or papules which can be found in skin folds and moist areas. It also often has satellite lesions, not found in the primary source of infection. It should be treated with nystatin (an antifungal) or a azole (a class of antifungals).
Impetigo
Staphylococcus aureus is the main cause although streptococcus. pyogenes is sometimes the pathogen. It is a superficial, spreading skin infection that manifests as yellow-brown crusts which may be bullous. It is commonest between the ages of 2-6 years and peaks in summer. It also occurs in close contact sports (e.g. rugby, wrestling) of any age. Treatment is with oral antibiotics.
Cellulitis
Cellulitis is an infection of subcutaneous layer of skin, which happens when the epidermis is broken and bacteria enter this layer below. This bacteria can be normal skin flora or foreign organisms but the most common organisms are group A streptococcus and staphlyococcus.
Clinical features can be those of a generalised infection - fever, nausea and headache - or found locally - itching and redness at the site of infection. The rash is usually red, macular, hot to touch and can be painful.
Treat with lots of antibiotics - it can be limb threatening. Use amoxicillin at first, or IV co-amoxiclav in more serious cases.
Chicken pox
Meningococcal sepsis
Scabies
Infestation with Sarcoptes scabiei'. Causes itching (fingerweb space; wrist flexure; axillae; abdomen, particularly around umbilicus and waistband; buttocks; and groin with itchy red penile and scrotal papules virtually diagnostic) and rash which is ulcerated and eczematous.
You need an anti-scabetic (malathion, permethrine). Apply from the neck down for 24h (yes it's a massive pain). Anybody who's been in the house needs to be treated as well. Everybody needs: to take a warm bath and soap up; scrub fingers and nails; apply anti-scabetic including soles of feet; wash only after 24h; use fresh bedsheets. You should then repeat this process a week later.