Y is for yellow fever, an infectious disease caused by a virus transmitted by mosquitoes, today found only in Central America, parts of South America and Africa. In severe cases the skin of the sufferer becomes yellow from jaundice - hence the name.

The infection causes fever, headache, nausea and nose bleeds and a slow heart rate, but most sufferers recover within about three days. In more serious cases, there is severe headache and pain in the neck, back and legs and damage to the liver and kidneys can occur. About 10 per cent of victims die.

No drug is effective against the yellow fever virus. Because of the slow heart rate, treatment is directed at maintaining the blood volume and transfusions are often necessary. But a vaccination that confers immunity for 10 years is available.

Y is also for yoga, a Hindu system of physical discipline and philosophy popular in the West that helps to maintain suppleness and is a useful relaxation technique. The main form of yoga practised here is hatha yoga, in which a series of poses, known as asanas, are adopted together with a special breathing technique.

If practised incorrectly, or by people in poor health, yoga can pose health hazards including back disorders and high blood pressure

Z is for Z-plasty, a technique used in plastic surgery to change the direction of a scar so that it can be hidden in natural skin creases or relieve skin tension. Z-plasty is particularly useful for revising unsightly scars on the face, and for releasing scarring across joints that restrict normal movement - on the fingers, or in the armpits.

A Z-shaped incision is made, with the central arm of the Z along the scar. Two V-shaped flaps are created by cutting the skin away from underlying tissue. The flaps are then transposed and stitched, creating a new Z shape that hides the original scar line and redistributes tension.