• Macule: This is a flat spot that can be seen but not felt. Macular rashes are usually pink or red (erythematous macules) and disappear (blanch) for a few seconds after pressing on them. This is because these macules consist of dilated, small capillaries. Rubella infection is one of the many causes of a pink macular rash. Pale or brown macules also occur, and are due to changes in the amount of pigment in the skin. They are often seen as part of the healing of damaged skin and do not disappear when pressed.
• Papule: This is a raised spot (a skin lump), which can be both seen and felt. A papule feels solid and can be any colour, although they are often pink or red or the colour of normal skin. Papular urticaria (allergic reaction to an insect bite) is one of the many causes of a papular rash. A nodule is larger than a papule.
• Vesicle: This is a small fluid filled blister that can be both seen and felt. The fluid in a vesicular rash is clear and may look like serum or water. If the vesicles leak, oozing of the fluid makes the skin wet (weepy). When the fluid dries it leaves a crust. If a vesicle bursts it leaves a small ulcer. Fever blisters (cold sores) and acute eczema are typical of a vesicular rash.
• Pustule: This is a small pus-filled blister. A pustular rash can be both seen and felt. If a pustule bursts it also forms a small ulcer, which may crust. Bacterial skin infection is one of the many causes of a pustular rash.

See the full-colour illustrations of skin conditions at the back of the book.

One type of rash often changes into another. For example, in chicken pox the rash starts as red macules, which develop into papules and eventually change into vesicles that heal leaving pale or dark macules or sometimes scars. Although identifying the type of rash does not necessarily give the diagnosis, it is an important step in limiting the range of possible diagnoses. Therefore a detailed description of the rash is needed.