(a) : Syphilis is caused by a spirochete (spiral bacterium) Treponema pallidum. The symptoms of syphilis occur in three stages. The first stage usually consists of a painless lesion called a chancre at the organism's site of entry. The second stage begins as the organism enters the blood. Symptoms such as fever, a flu like illness, a skin rash, hair loss, and swollen joints may come and go over a period of several years. In the third stage permanent brain damage, heart disease, and blindness often occurs. AIDS is a viral disease caused by Human Immuno deficiency virus. Gonorrhoea is a sexual disease and its causative organism is Neisseria gonorrhoed. Typhoid is caused by bacillus bacteria Salmonella typhi.