Drinking milk will have no bad effect on the health as it is a source of almost all nutrients and it is recommended for good health.

A person will be healthy if he eats a proper diet (option 3), lives in a hygienic environment (option 4) and keeps his body fit by exercising regularly (option 1). Thus, options (1), (3) and (4) help a person to be healthy as these habits keep the germs away and thus, reduce the chances of getting infected by diseases. But, drinking alcohol is not a healthy practice as it adversely affects the nervous system and its prolonged consumption adversely affects the liver and general body health. Thus, (2) is the correct answer because drinking alcohol is responsible for the ill health of a person.

A defective gene that leads to a defect is a factor which is present inside the body cells of an organism. Hence, it is an internal factor. Extrinsic factors are those which are outside of the body and they include causative agents, diet, community, social system, etc.

 The AIDS virus does not spread through handshake. 

Most of the skin infections are caused by fungi. For example, ringworms. Bacteria and viruses cause internal infections like TB, influenza, etc. Mosquitoes can cause rashes on skin (it is not a disease) and may also transmit other diseases (not related to skin). Thus, (1), (2) and (3) are not suitable. So, only (4) is correct.

Vaccines are 'preventive', i.e. they help us in fighting against a disease before it is actually caused. The only option that suits this fact is (3). On the contrary, options (1) and (2) are related to treatments, i.e. cure after the host is infected. This is done by using suitable medicines and drugs. Hence, options (1), (2) and (4) are incorrect.

Health refers to physical, mental and social well-being. Thus, absence of any tension, anxiety (mental factors) and social problems contribute to health. But absence of work is not a factor that can make us healthy. Hence, (2) is the correct answer.

Common cold, pneumonia and tuberculosis are airborne diseases, i.e. they spread through air because their causative agents, i.e. virus and bacteria, are present in the air. On the contrary, malaria is not airborne because it is caused by Plasmodium, which spreads through the bite of female anopheles mosquitoes and is not present in the air. Thus, (4) is the correct answer.

Bacterial diseases are those diseases that are caused and transmitted by different bacteria. Typhoid, cholera and tuberculosis are caused by bacteria namely Salmonella typhii, Vibrio cholera and Bacillus calmettie, respectively. Dengue fever is caused by a virus, which is transmitted by the vector Aedes mosquito. Hence, option (3) is the correct answer.

Common cold (usually caused by viruses) is an infection of the upper respiratory tract. Signs and symptoms include coughing, sore throat, runny nose, sneezing and fever which usually resolve in seven to ten days, with some symptoms lasting up to three weeks. 

Headache, diarrhoea and common cold last for some time and affect us severely for that short period. But they do not leave any of the effects like weakness. Thus, these are acute diseases. On the contrary, tuberculosis is chronic because it affects us for a long time and also makes the body weak, even after the infection is cured. It is caused due to bacteria and in this disease, the lungs and the respiratory system are affected. In this, mucus begins to collect in lungs, gradually leading to inefficient gaseous exchange, frequent cough and even bleeding in the throat at later stages. Hence, option (4) is the correct answer.

Diarrhoea is an infection of stomach (gastro) and intestine in which the person suffers from water loss and loose stools. Hence, it is a gastrointestinal disorder (as is clear from its name).