Cancer causing agents are called carcinogens. They are physical, chemical agents, ionizing radiations and biological agents.
(i) Physical Irritant: Heavy smoking causes lung cancer and cancers of oral cavity, pharynx (throat) and larynx. Betel and tobacco chewing causes oral cancer. Excessive exposure to sunlight may cause skin cancer.
(ii) Chemical agents: Nicotine, caffeine, products of combustion of coal and oil, pesticides, asbestos, nickel, certain dyes, and artificial sweeteners induce cancer.
(iii) Radiations: Ionizing radiations like X-rays, gamma-rays, radioactive substances and non-ionising radiations like UV rays cause DNA damage leading to cancer.
(iv) Biological agents: Cancer causing viruses are called oncogenic viruses.

Symptoms: Diabetes mellitus is associated with several metabolic alterations. The most important symptoms are.
(i) Increased blood glucose level (Hyperglycemia)
(ii) Increased urine output (Polyuria) leading to dehydration.
(iii) Loss of water leads to thirst (Polydipsia) resulting in increased fluid intake.
(iv) Excessive glucose excreted in urine (Glycosuria).
(v) Excess hunger (Polyphagia) due to loss of glucose in urine.
(vi) Fatigue and loss of weight.

Prolonged use of alcohol depresses the nervous system, by acting as a sedative and analgesic substance. Some of the harmful effects are:
(i) Nerve cell damage resulting in various mental and physical disturbances.
(ii) Lack of co-ordination of body organs.
(iii) Blurred or reduced vision, results in road accidents.
(iv) Dilation of blood vessels which may affect functioning of the heart.
(v) Liver damage resulting in fatty liver which leads to Cirrhosis and formation of fibrous tissues.
(vi) Body loses its control and consciousness eventually leading to health complications and ultimately to death. 

Behavioural Changes:
Adverse effects among adolescents are:
(i) Drop in academic performance, absence from school or college.
(ii) Lack of interest in personal hygiene, isolation, depression, fatigue and aggressive behaviour.
(iii) Deteriorating relationship with family and friends.
(iv) Change in food and sleeping habits.
(v) Fluctuation in body weight and appetite.

Instructions to be given by parents and teachers to the child are.
(i) Do not talk to any suspected person or strangers and to maintain a distance.
(ii) Not to be alone with any unknown person.
(iii) To be careful while traveling alone public or private transport.
(iv) Not to receive any money, toys, gifts or chocolates from person known or unknown to them without the knowledge of their parents,
(v) Not to allow any known or unknown person to touch them.