Welcome to the worldWelcome to the world
of biologyof biology
Why do we fall ill???Why do we fall ill???
What is Disease??
• Disease is any condition that may lead to
discomfort,distress,health problems, or
death of the affected person…….
Health and It’s failure
• HEALTH is described as the state of complete physical, mental
and social well being.Thus,being healthy means that one feels
good physically, has a positive outlook and is able to cope with
the social and mental pressures without much difficulty .A
person should maintain the good hygiene to reduce the chances
of developing infection.
• A doctor is able to diagnose disease by identifying the
symptoms which may be physical. mental or both.
• Our health is affected not only by unbalanced diet but also by
disease causing organisms which may be water borne, air borne
or food borne and some diseases are caused by infection
through microorganisms, insects organisms such as
• Infectious diseases spread through
infectious agents (Virus,Bacteria,other
• An infectious disease is called
communicable disease, if it spreads from
an infected person to a healthy person..
• Disease caused by non infectious agent
(Excessive weight, genetic defects,etc.)
are called Non-infectious diseases….
• Non-infectious disease can’t be transferred
from a diseases person to a healthy
Modes of transmission of diseases
• Infectious disease(bacteria,fungi,etc.)
• Contaminated food or water
• Blood transmission
• Direct physical contact
Definitions of types of diseases
(i) Acute diseases-Diseases are described as being acute, when their symptoms
are quickly visible in the body.
(ii) Chronic diseases- These types of diseases are long-term, with the symptoms
lasting for months or years.
(iii) Congenital diseases-These diseases are present since birth. They are caused
due to genetic abnormalities or due to metabolic disorders or malfunctioning
of any organ.
(iv) Acquired diseases-These diseases are those which develop after birth. These
are classified into two types-(i) communicable ; (ii) non-communicable.
(v) Infectious diseases-These diseases are caused by biological agents or pathogens
such as virus, bacteria and fungi. These diseases can rapidly spread from one
person to another.
(vi) Non-infectious diseases-These include organic or metabolic diseases. These
diseases are restricted only to those persons who are suffering from them.
I. Infection Agents
Organisms that can cause diseases are classified in wide
range of categories. Some of them are viruses, some
are bacteria, some are fungi and some are unicellular
animals, the protozoans.Some diseases are also
caused by multicellular organisms such as worms.
Common example of diseases caused by viruses are the
common cold,influenza,dengue,fever and AIDS and
common skin infection caused by different kinds of
fungi.Protozoans cause many familiar diseases such as
malaria and kala azar.Worms tend to cause a variety of
intestinal infections and elephtiasis
II.Means of Spread of
Infectious diseases spread from one infected
person to other normal persons by various
1.Air-borne diseases,e.g., common
cold,pneumonia.such disease-causing microbes are
spread through out the air..
2.Water-borne diseases,e.g., cholera.Infectious
diseases can also spread through water.These
occur when a stool from someone suffering from an
infectious gut disease,such as cholera or
amebiasis,gets mixed with drinking water used by
diseases,e.g.,AIDS.These diseases are
transmitted by sexual contact from one
partner to the other partner..
4.Spread of diseases through vectors.Many animals
which live around us may carry diseases.These
animals can transfer infecting agents from a sick
person to another potential host..
Example of Diseases Caused to
Infection By Microorganisms
Malaria is a fatal disease for human beings. Approximately 300 million people
around the world get infected with malaria every year.Malaria is caused by a
protozoan parasite Plasmodium.This disease spreads through the bite of an
insect vector-the female Anopheles mosquito which feeds on the human blood.
Symptoms-1.Cold stage,feeling of extreme cold and shivers ;2.hot stage,high
fever, faster respiration and heart beat; and 3.sweating stage,due to the
profuse sweating,temperature of the body goes down to norrmal.Even after
cured of malaria, the patient feels weak and anaemic.
Prevention-1.Insect-repellents should be used to prevent mosquito bites.
2.One should keep under mosquito nets.
3.Wire-gauze should be used on doors and windows of
houses to prevent entry of mosquitoes.
4.Mosqito larvae can be killed by sprinkling kerosene oil on
large-sized water bodies.
5.The breeding grounds should be should be
destroyed.Thus,the ditchess,puddles or swamps around human
dwellings should be drained or filled.
Control-A drug named quinine, which is extracted from the bark of the
Cinchona tree,is used to treat a person suffering from malaria.
Diseases Caused by Viruses
Influenza is commonly called flu. It is caused by an influenza virus.
There exist three types of inflenza viruses-A,B and C. A and B are
important because these are responsible for causing of disease
epidemics throughout the world. The inhaled virus attacks the
epithelial cells in the mucous membrane of nose, throat and upper
respiratory tract. Influenza is spread mainly from person to person
contact and by droplet infection via sneezing, coughing or talking
Symptoms-Onset on chills, discharge from
of respiratory mucosa and general weakness.Fever lasts for three
days in adults.
Prevention-We should try to keep away from flu patients.
Control-There is no effective control of ifluenza.However,vaccines are
used for the control of infection and antiviral drugs are used to cure.
Rest speeds up the recovery
The bite of a rabid dog(mad dog) and some other such as
monkeys,cats or rabbits may cause rabies,which is fatal viral
disease cause by rabies virus or lyssa virus which is present
in the saliva of infected animals.The viruses are injected into
human blood by the bite of rabid animal.
Symptoms-Rabies is characterised by serve,headache,high
fever,painful contraction of muscles of throat and chest.The patient
feels restless,does excessive salivation, has a choking feeling and
finds difficulty in take of even liquid.
Prevention-1.Cleaning the wound with carbolic soap and clear water
immediately after the dog bite is an important preventive measure.
2.Any antiseptic medicine should be applied the wound
caused by a dog bite.
3.A rabid animal shows excessive salivation and
tries to seek isolation after bite. Such mad animals should be
Control-Rabies can be treated with Pasteur’s treatment,in
which a course of 14 vaccines was given.Currently 5 anti-
rabies vaccines are prescribed at an interval of 0-3-7-14-30
day of dog bite.
The disease of AIDS is caused by retrovirus known as HIV.AIDS virus
attacks white blood cells of human beings and weakens the human
body’s immunity or self-defense meachanism.Since AIDS virus
reduces the natural immunity of the human body,therefore,the
patients suffering from AIDS becomes prone to many other infections
or diseases.In fact, the patients suffering from AIDS die from other
Symptoms-Some important symptoms of AIDS are following: swollen
lymph nodes; regular fever;sweating at night and weight loss. AIDS
virus causes severe damage to brain and may lead to loss of
memory, ability to speak and of clear thinking.
Prevention-So far no medicine or vaccine has been developed to cure
AIDS. The following steps may help in controlling infection of this
1.People should be educated about AIDS transmission.
2.Disposable syringes and needles should be used for injection.
3.Sexual contact with unknown people should be avoided.
4.Sterillised needles may be provided to drug addicts.
5.Before receiving blood for transfussion,one should ensure that it
has been screened for HIV.
Diseases caused by Bacteria
Cholera is an actuely infectious, fatal disease.Sometimes cholera
occurs in epidemic form and spread over large
population.Cholera is caused by the bacterium, Vibrio cholerae.It
is transmitted by flies,contaminated food and water.When the
bacteria f cholera is ingested,they multiply in the small intestine
and invade its epithelial cells.When the bacteria die,they release
toxins which irritate the intestinal lining and lead to the secretion
of large amounts of water and salts.
Symptoms-1.Watery diarrhea which is generally painless.
2.Efforts vomiting without nausea.
3.Loss of several liters of liquid from the patient’s
body takes place within hours.
Prevention-1.Person should be immunised by standard cholera
2.Careful personal hygiene and good sanitation in the
community are the only certain protection against cholera.
3.In cholera-prone areas, boiled water and cooked
should be taken
TyphoidTyphoid is the most common communicable disease in India. It is
common in humans of the 1 to 15 years age group. Typhoid is cause
by a rod-shaped and motile bacterium, called Salmonella typhi which
is found in the intestine of intestine of human beings.Infection takes
place by the oral route through ingestion of food,milk or water
contaminated by contact with faecal mattar of the typhoid patient.The
bacteria spread through faecal mattar by house flies.
Symptoms-1.Headache and typhoid fever which rises maximum in the
afternoon. The temperature increases each day in the first week.
2.High fever in the second week.Fever gradually declines
3.In nutshell symptoms of typhoid include continuous
fever often with delirium, slow pulse, tender and distended
abdomen,diarrhoea with water-green stools.
Prevention-1.Proper sanitation and disposal of faecal mattar
2.TAB-vaccination provides immunity for 3 days.
3.Typhoral oral vaccine also prevents typhoid.
Control-There are standard drugs e.g., Chloromycetin which cure