A state of complete physical, mental & social well - being. A healthy person is more efficient at work. Increases productivity & brings economic prosperity.

Health is a state of complete physical, mental and social well-being, and not merely an abence of disease, or infirmity. Two ways of maintaining helath are -
(i) By taking hygienic balanced diet,
(ii) by taking clean drinking water

(i) Cancer (Malignancy). It is an abnormal form of development which occurs during adult life and converts normal cells into abnormal cells called cancerous cells.
(ii)Carcinogenic.The factors which are considered to cause cancer are known as carcinogenic e.g. Chewing of pan and tobacco.

HIV.Human Immuno deficiency Virus.

Subgroup.Lentivirus

Family.Retroviridae

Oil and sweat are secreted by sebaceous glands and sweat glands respectively.They are acidic in nature  \({ p }^{ H }3to5\quad \)They inhibit the growth of micro-organisms on the surface of skin.Sweat also contains an enzyme lysozyme which destroys the cell walls of many bacteria,thus help in protection. 

1.First line of defence is physical barrier such as skin, oil and sweat,tear, network or fine hair in nostrils,ciliated epithelium of respiratory tract.
2.Second line of defence. Various chemicals are associated with this defence mechanism.It is also termed inflammatory response.it may be localized or systemic in which body temperature rises with the release of pathogens and number of WBC increases.

Cell Mediated Immunity:
(i) It is mediated by T-lymphocytes.
There are two group of T-lymphocytes.
(i) Cytotoxic or killer T cells,which kill the specific target cells by a variety of mechanisms.
(ii) Helper T cells,which activate the specific B-cells to produce antibodies.
T-lymphocytes are responsible for graft rejections.

Immunity is defined as the ability of the body to defend itself from most of the foreign agents.
Its two broad categories are:
(i) Innate Immunity;and
(ii) Acquired Immunity.

Control of infectious diseases:
(i) Periodic cleaning and disinfection of water reservoirs, pools, tanks and observing standard practices of hygiene in public catering, for preventing those diseases that spread through food and water.
(ii) Consumption of clean drinking water, food, vegetables and fruits.
(iii) Many infectious diseases can be prevented by vaccination.
(iv) For vector-borne (mosquito is the vector) diseases, eradication/elimination of vectors and their breeding places could help in their control.

Two special types of lymphocytes in humans are B-lymphocytes and T-lymphocytes. B-lymphocytes execute antibody-mediated response while T-lymphocytes show cell-mediated response.

B-lymphocytes and T-lymphocytes

Both these types of cella multiply and produce a clone of cells.

B-lymphocytes produce an array of proteins,called antibodies.

The T-lymphocytes stimulate the B-cells to produce antibodies.

Memory B-cells and T-cells remain inthe body,for secondary immune response,in case of subsequent encounter by the pathogen.

Many secondary metabolites obtained from plants are used for their medicinal properties, e.g. opiates derived from poppy plant are used as drugs to relieve pain. 
The problem starts when these metabolites are misused for having pleasure and are taken in doses above the prescribed limit, e.g. opium addict loses weight, fertility and interest in work.