Haemophilia is genetic illness suffered by men causing the blood not to clot when the skin is broken. This can be very dangerous, as even small cuts will continue to bleed, causing massive loss of blood unless treated properly.
Haemophilia can be controlled quite succesfully today by regular injects of clotting factor, Factor 8.
The reason that the illness is only suffered by men is due to it being inherited from the mothers X-chromosone. Women have two X-chromosones whereas men have one X and one Y. It does not matter if their is a defect in a womens X-chromosone as the equivalent allele in her other chromosone would express itself and she would not have the illness. However the Y-chromosone in men is shorter and has a few genes missing. If the recessive gene responsible for blood clotting in a man's X-chromosone is deficicient their is no equivalent on the Y-chromosone, so the deficient gene must become dominant and he will develop the illness.
This means also that although women can be carriers of the illness, if men have the gene they also have the illness.