Grade 9
Why Do We Fall Ill
Health
Health - A state of physical, mental, and social well-being, which includes a unity
and harmony within the mind, body, and soul of an organism
Diseases
Disease - Any condition that can lead to discomfort, distress, health problems, and
even death of the affected person
Symptoms - Indications of disease, such as headache, stomach pain, nausea, etc.;
can only be felt by the patient
Signs of a disease include fever, vomiting, diarrhoea, etc.; can be observed by a
physician
Incubation period - The time interval between infection and appearance of
symptoms
Causes of diseases
On the basis of its duration - Acute and Chronic
Acute - Lasts for a short period of time, eg. cold, cough, influenza, etc.
Chronic - Lasts for long periods of time, eg. diabetes, kidney stones,
etc.
On the basis of causative agents - Infectious and Non-infectious
Infectious - Diseases such as influenza, cold, etc., which are caused
due to infectious agents
Non-infectious - Diseases such as high blood pressure, cancer, etc.,
which are caused by some internal causes such as excessive weight,
genetic defects, etc.
Infectious agents
They are the disease-causing microorganisms which belong to different categories such
as:
1. Viruses - These are tiny organisms that grow, multiply, or reproduce only inside
the host cells
Diseases caused by viruses - Influenza, cold (Rhinovirus), dengue, AIDS,
etc.
2. Bacteria - These are unicellular organisms; larger than viruses
Diseases caused by bacteria - Whooping cough, typhoid, cholera, anthrax,
etc.
3. Fungi - These are plant-like organisms; heterotrophic
Diseases caused by Fungi - Athlete’s foot, candidiasis, ringworms, etc.
4. Protozoa - These are simple, primitive unicellular organisms which are often
found in water
Diseases caused by Protozoa - Amoebiasis, kala azar, malaria, African
sleeping sickness, etc.
5. Multicellular animals like worms - These are parasites that infect the
intestines of human beings and other animals
Diseases caused by worms- Diarrhoea, anaemia, liver rot, etc.
Communicable diseases
An infectious disease is classified as communicable disease when it can be
transferred from an infected person to a healthy person
Means of disease spread
They are the disease-causing microorganisms which belong to different categories such
as:
1. Air-borne diseases - Transmitted when disease-causing microorganisms are
expelled into the air by coughing, sneezing, talking, etc.
Eg. common cold, chicken pox, small pox, pneumonia, influenza,
tuberculosis, etc.
2. Water-borne diseases - Spread when the excretions, from an infected
person, containing causal microorganisms get mixed with drinking water and
this contaminated water is consumed
Eg. cholera, typhoid, hepatitis A, etc.
3. Physical contact – Includes sexually-transmitted diseases.
Eg. syphilis, gonorrhoea, AIDS, etc.
4. Blood to blood contact – Such contact is established during blood
transfusion or pregnancy (between the mother and her baby)
Eg. AIDS can spread through blood contact
5. Animals – Animals which transfer disease-causing microorganisms from an
infected person to others are called vectors
Eg. female mosquitoes can transfer the malaria-causing Plasmodium
Effects of diseases
Local effects - Includes swelling, pain, joint stiffness, etc., that occur only at
the site of infection
General effects - Includes fever chills, headaches, fatigue, loss of appetite,
etc., that occur all over the body
Inflammation- The process by which the body’s immune system shows
response to protect the body from infection
Prevention of diseases
Immunisation - The protection of the body from communicable diseases by
administration of some agent that mimics the microbe
Antigens - Foreign substances that invade our body
Vaccines available against tetanus, polio, measles, hepatitis B, etc.