(a) AIDS: AIDS stands for acquired immunodeficiency syndrome, which is caused by a retrovirus known as human immunodeficiency virus. HIV infects the host body by attacking the WBCs, thereby weakening the body's self defence or immune system.

AIDS is transmitted from one person to another by the following modes:

(i) Unprotected sexual contact with the infected person.

(ii) Transfusion of blood contaminated by HIV.

(iii) Use of unsterilised needles for injections.

(iv) From an infected mother to her child during pregnancy.

Prevention of AIDS:

(i) Avoiding sexual contact with an unknown person

(ii) Using sterilised needles for injections

(iii) Screening the blood for HIV before transfusing it to some other person So far, there is no medicine to treat AIDS.

(b) Malaria: Malaria is an infectious disease that is caused by Plasmodium. It spreads by the bite of a female Anopheles mosquito that carries the malarial parasite. Malarial infection spreads in stagnant water bodies because the female mosquito breeds in such places. It causes headache, nausea, muscular pain and high fever in the infected person. Malarial parasite attack is of 6-10 hours of duration with three stages, i.e., cold stage (extreme cold and shivering), hot stage (high fever) and sweating stage ( profuse sweating).

Malaria can be prevented by the following ways:

(i) Using insect repellents to avoid mosquito bites.

(ii) Sprinkling kerosene oil on large water bodies.

(iii) Introducing Gambusia in water bodies, as it feeds on mosquito larvae.

(iv) Spraying insecticides to kill adult mosquitoes Quinine is a drug that is used to treat a person suffering from malaria.