Purple or red, flat, pinhead-sized spots that arise in the skin or mucus membranes. They do not blanch (go pale) when pressure is applied to them. Petechia are caused by a localised haemorrhage (leakage of blood) from small blood vessels. They arise in purpura (a group of bleeding disorders) and, occasionally, in bacterial endocarditis (infection of the inner lining of the heart). Petechiae can also occur after periods of excessive straining; in conditions such as pertussis (whooping cough), for example.