What is a disease?
Any condition which interferes with the normal functioning of the body is called
a disease. In other words, disease may be defined as a disorder in the physical,
physiological, psychological or social state of a person caused due to nutritional
deficiency, physiological disorder, genetic disorder, pathogen or any other reason.
Types of Diseases
Congenital disease : The disease which is present from birth (e.g. hole in the
heart in infants). They are caused by some genetic abnormality or metabolic
disorder or malfunctioning of an organ.
B. Acquired disease : The disease which may occur after birth during one’s
Acquired diseases may generally be classified into :
Infectious diseases : The diseases which can be transmitted from person to
person e.g. measles.
(ii) Degenerative diseases : The diseases caused by the malfunction of some vital
organs of the body e.g. heart failure.


(iii) Deficiency diseases : These are caused due to nutritional deficiency such as
that of minerals or vitamins in the diet e.g. anaemia (Fe, Beri- beri (vitamin
B). You have read about such diseases in an earlier lesson 27.
(iv) Cancer : This is an abnormal, uncontrolled and unwanted growth of cells. e.g.
breast cancer.
Acquired diseases are studied under two categories (Table 28.2).
Communicable diseases : The diseases which can be transmitted from
an infected person to a healthy person.
Non-communicable diseases : These diseases do not spread from an
affected person to a healthy person.
Table 28.2 Differences between communicable and
non-communicable diseases
Communicable diseases
Non-communicable diseases
Caused by some biological agents or
pathogens, such as viruses, bacteria,
protozoans, helminths (worms) etc.
Spread from one person to another
through contact, water, air, food, etc.
The concern of society as these are
related to community health.
28.1.2 Modes of Spread of Communicable Diseases
Communicable diseases spread from the infected person to a healthy person in the
following ways.
Direct transmission
The pathogens of diseases infect a healthy person directly without an intermediate
agent. It can take place by various means such as,
Direct contact between the infected person and the healthy person :
Diseases like small pox, chicken pox, syphilis, gonorrhoea spread through
direct contact.
(ii) Droplet infection : The infected person throws out tiny droplets of mucus by
coughing, sneezing or spitting. These droplets may contain the pathogen. By
inhaling the air containing the droplets, a healthy person may get the infection.
Diseases like common cold, pneumonia, influenza, measles, tuberculosis and
whooping cough spread through droplet infection.
Caused to some specific factor, such
as malfunctioning of some vital organ,
deficiency of nutrients, etc.
Do not spread from one person to
another by contact.
The concern of the individual only.
Environment and Health
Some Common Human Diseases
(iii) Contact with soil contaminated with disease-causing viruses, bacteria etc.
(iv) Animal bite : Viruses of rabies are introduced through the wound caused by
the bite of rabid animals, especially dogs. The virus is present in the saliva
of the rabid animals.
Indirect transmission
The pathogens of certain diseases reach the human body through some intermediate
agents. It can take place by various means, which are as follows :
By vectors such as houseflies, mosquitoes, and cockroaches. Examples:
Houseflies carry the causative organisms of cholera on their legs and mouth
parts from the faeces and sputum of infected persons to food and drinks and
contaminate them. When this contaminated food is taken by a healthy person,
he gets the infection. Similarly, mosquitoes carry virus of dengue and malarial
parasite which causes malaria.
(ii) Air-borne : The pathogens may reach humans with air and dust. The epidemic
typhus spreads by inhalation of dried faeces of infected fly.
(iii) Object borne (Fonite borne) : Many diseases are transmitted through the use
of contaminated articles, such as clothes, utensils, toys, door handles, taps,
syringes and surgical instruments, etc.
(iv) Water borne : If potable water (drinking water) is contaminated with
pathogens of diseases such as cholera, diarhhoea, hepatitis or jaundice, it
reaches a healthy person upon consuming such water

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