Erythema:

Erythema is one of  the,commonest primary lesions of  the skin. It  is produced by  the dilation of the cutaneous blood vessels.  If the dilation  is accompanied  by increased permeability, redness and oederna of  the skin  is produced and features of urticaria1 eruption are seen. At  times the cutaneous blood vessels are so  irritated and  damaged, that they permit the whole blood  to pass through, as  in  purpuras (haemorrhagic  rashes).  Thus,  it will  be  seen that  these three lesions, erythema, urticaria and  purpura, depend  upon  the degree of stimulation or  damiige to the cutaneous vascular system. 

Diaper Rash (Napkin Rash):

In  infants it affects  the areas of skin covered  by  the napkin. Lesions  consist mainly of simple erythema, though they may become vesicular or even ulcerative. They are situated, most frequently, on  the prominences, and may occupy the inner parts of the thighs, ,perineum and genitalia.  Usually the flexures are not  affected.  The causes of  napkin rash are wet or  soiled napkins, soap  left  in  the napkins after washing, strong ammonical urine and poor general  health.