CBSE Class 9 Science Why Do We Fall Ill Notes Set B



Every living organism, may be plant or animal, requires food (nutrition) for its survival, maintenance, growth and development. Nutrition is required in specific amounts. Proper dietary habits lead to sound health and proper mental development. A person is said to be healthy if one: 

  1. has no symptoms of disease and anxiety.
  2. has no physical deformity.
  3. has no mental problems and social tensions.
  4. has no psychological tensions.
  5. has all the body organs functioning properly.
  6. has purposeful life.
  7. has sufficient balanced diet.

The most widely accepted definition is (1947) World Health Organization’s description that states “Health is a state of physical, mental and social well being and not merely the absence of disease or infirmity”.

(a)Significance of Health :

Good health is “health body with a healthy mind and healthy attitude”. Good health increases our efficiency for doing work. The increased efficiency of a man due to good health contributes to his own progress, the progress of community and the progress of nation as a whole. Good health also make a man happy and cheerful. It allows a person to have the initiative for betterment. It makes living a joyful experience. It keep a person not only happy but also provides cheerfulness to the people. Good health is a condition for our purposeful existence in this world.

(b)Community and Personal Health :-

Health of a person depends upon his personal habits as well as his environment. One cannot remain healthy, if his environment is not clean. Good economic condition and job are needed for individual health, so that the person can have balanced diet and be healthy. Social equality and harmony in the community are also important to maintain individual’s health. If neighbors of a person are healthy, but possess no civic sense (i.e., they are in the habit of throwing garbage in an open space and there), files, mosquitoes, and other disease causing microorganisms will grow on the filth (foul matter) thrown by them. These microorganisms will infect nearby foodstuffs thereby spreading diseases. Thus, a person with his clean home, but unclean neighborhood will become sick. Similarly, the health of the whole community depends on the personal habits of various individuals who constitute the community. So community health can be defined as “All the personal health along with the environmental services for the improvement of health of community.” It comprises of all efforts for maintaining, protecting and improving the health of the people. WHO is doing remarkable work in community health.

(c)Difference Between Personal Health and Community Health :


The various activities involved in maintaining community health are as follows :

(i)Maintaining proper sanitation of the environment by :

  1. Providing cleans and safe drinking water.
  2. Providing good sewage and rain wate disposal systems (through underground pipes).
  3. Proper garbage disposal.
  4. Strict enforcement of antipollution laws, management of different types of environmental pollution by Central and State Control Boards.

(ii)Providing proper facilities for prevention and control of diseases such as :

  1. Preventive vaccinations against a number of diseases like tuberculosis, diphtheria, whooping cough, tetanus, measles, hepatitis, polio, mumps, etc.
  2. Spraying mosquito and germ killing chemicals (insecticides, pesticides etc.) at regular intervals.

(iii)Providing health education : to people about the mode of transmission of diseases and mechanism to control communicable diseases; importance of balanced diet; effects of bad habits Like alcoholism, addiction, etc.

(iv)Establishment of health care services: primary Health Centres, District Hospitals, Community Health Centres, Medical Colleges, All India institutes, Regional Hospitals, etc.

(v)Prevention of food adulteration.

(vi)Providing maternity and child care centres: so that mortality rate among children is reduced to a great extent. Provision of family planning advice and medical care to school going children.


A disease is a condition of the body or a part of it in which functions are disturbed. Disease may also be defined as morphological (structural), physiological (functional) or psychological disturbance in the body of body parts caused by external agencies which may be nonparasitic e.g. Deficiency of nutrients or may be parasitic e.g. Caused by vieuses, bacteria, fungi, etc. The term disease means dis-ease or discomfort or without ease. In short, it can be defined as “disease is disorder of body.”

(a) Distinction Between Healthy and Disease Free :

The term disease is used when we find a specific and particular cause for discomfort. We may not be knowing the main cause of the discomfort, but still we can use the term disease. A person may not be suffering from any disease but may be in poor health. This is particularly true for social and mental health, where we can be in poor health without there being a cause in the form of an actual disease. This is the reason why, when we think about health, we think about societies and communities. On the other hand, when we think about disease, we think about individual sufferers.


(b) Manifestation of Diseases :

There are number of tissue in the body, which aggregate together to form organ while a number of organs make up an organ system. Each organ system is performing a specific function. Each organ in the organ system also has a specific role to play. For example, in digestive, system, teeth help in mastication, stomach and intestine help in digestion, kidneys take part in excretion, bones and muscles hold the body parts together to form a musculoskeletal system that helps the body to move. When a person is suffering from any disease, then the physiological processes (functioning) or the appearance of organs in organ systems will change. These changes give rise to symptoms and signs of disease. symptoms are evidences of the patient’s feeling of being wrong. For example, headache, loose motions or a wound with pus are symptoms which may indicate the occurrence of discomfort. Headache may be due to examination stress, meningitis. The symptoms give an indication of the presence of a particular disease. The physicians will also get laboratory tests done to identify the disease further.

(c) Acute and Chronic Diseases :

The manifestations of diseases are different depending upon a number of factors. one of the factor is     duration of disease. on the basis of duration serious disease can be acute or chronic.

  1. Acute disease : actual disease is the one which has a short duration by relatively severe course. most people with acute illness can expect to return to normal health. a case of cough and common cold is an example of an acute illness which lasts only for a few days. afterwards the patient becomes well without any bad effect, loss of weight, feeling of tiredness or short of breath.
  2. Chronic disease : chronic disease is the one which is long lasting is usually slow to develop, often having a major effect on health, reducing the person’s ability to do work efficiently, learning in school o r doing work. the patient will also weight and feel tired all the time. examples of chronic diseases include tuberculosis, diabetes, asthma, hypertension, kidney disease, depression, etc. in these diseases we can develop a treatment plan to manage symptoms and prevent complications with the help of doctor.


(d) Causes of Diseases :

The various causes of diseases are

(i) Pathogens : They are disease causing organism like bacteria, viruses, fungi, protozoans, worms, etc. The pathogens are transferred to human being through air, contaminated food, water, soil and animals. Pathogens are primary cause of infectious diseases. However, every body does not suffer equally from infectious agents. There are some contributory causes that increase the proneness of an individual to catch the disease. 

(ii) Lack of nutrition diet: it is a second level cause of disease as absence of nutritious diet makes a person unhealthy. Unhealthy persons are susceptible to various diseases in comparison to healthier persons. Another contributory cause can be poor heredity which increases proneness of individual to a particular disease.

(iii) Lack of public services: Government should provide clean drinking water, good sewage disposal, proper garbage disposal, etc. If the public services are poor, there are more chances of contamination of food and water. They are the third level cause of disease. Poor people, due to poverty, live in unclean surroundings where even basic amenities are lacking, there are three level causes of diseases. These are infection with pathogen (1st level), lack of nutrition diet and poor heredity (2nd level) and lack of public services (3rd level).


  1. Congenital Diseases : Congenital diseases are present right from the birth. They are caused either due to genetic disorders or environmental factors during development or due to combination of these factors. These diseases pass on from generation to generation e.g. hemophilia, colour blindness, sickle cell anemia, Down ’s syndrome, albinism etc.
  2. Acquired Diseases : These disease are acquired by an organism after birth and are not inheritable i.e., do not pass on from one generation to another. These are futher classified into categories; 
  1. Communicable or infectious diseases: These diseases are caused by pathogens/infectious agents such as bacteria, viruses, fungi, protozoans, worms, etc. These diseases can spread from diseased person to healthy parson by means of air (droplet method), water, food, insects, physical contact, etc.,e.g. tuberculosis, malaria, diarrhea etc. 
  2. Non-Communicable or Non-Infectious diseases: These diseases can’t be spread through infected persons to healthy persons. e.g. Scurvy



 (a)infectious Agents :

The various infectious agents are-bacteria, viruses, protozoans, helminthes (worms) and fungi.

  1. Bacteria : They are unicellular, prokaryotic, microscopic organism. they reproduce very quickly. Some common diseases caused by bacteria are typhoid, cholera, tuberculosis, anthrax, diphtheria, tetanus, etc.
  2. Viruses : They are submicroscopic organisms. They cannot reproduce by  themselves because they do not have their own metabolic machinery. They utilise the metabolic machinery of the host cell and multiply. The various diseases caused by viruses are common cold, influenza, dengue fever, AIDS , measles, mumps, polio, small pox, chicken pox, etc.
  3. Protozoans : They are microscopic unicellular, eukaryotic organisms. The various diseases caused by protozoa are malaria (caused by Plasmodium), kala-azar (caused by Leishmania), etc.
  4. Helminthes : Helminthes are multicellular worms which are mostly present in intestine. They cause taeniasis (caused by tapeworm), ascariasis (caused by worm), elephantiasis (caused by filariasis worm, hence also know as filariasis), etc.
  5. Fungi : They are also multicellular, eukaryotic, heterotrophic organisms. They cause ring worm, athlete’s foot and other skin infections.

(b)Reason for Categorization of infectious Agents :

it is important to categories infectious agents because each group of organisms have some common traits and many similar biochemical pathways. As a result, a drug that blocks one of the biochemical pathways peculiar to one group would be effective against many members. Antibiotics are used for killing bacteria. They inhibit wall synthesis (e.g. penicillin), inhibit ribosome function (e.g. erythromycin, streptomycin) or DNA replication (e.g., ciprofloxacin). However, antibiotics are not effective against viruses. Protozoan infections are treated by different types of drugs. Antifungal drugs are useful against all types of fungi. Vermicides are used for overcoming worm infection.

(c)Means of Spread :

Infection diseases are called communicable diseases because they can spread   from affected persons to a healthy person. The means of communication or spread are different for different pathogens.

(i)Direct transmission. The pathogens are transmitted from an infected person to a healthy person directly without an intermediate agent. It occurs in the Following ways:

  1. Contact with infected person ; Diseases like chicken pox, small pox, ring worm are spread by actual contact between infected person and a healthy person. Such diseases are called contagious diseases. The sexual contact is one of the closest physical contacts two people can have with each other. Diseases like syphilis, gonorrhea (both caused bybacteria0 and AIDS (caused by virus) are transmitted by sexual contact from one partner (infected) to the other (healthy).
  2. Contact with Soil : The infectious agent of tetanus can enter the human body from soil through injuries.
  3. Animal bites : The rabies virus is injected in the human body by the bite of rabied dog or monkey.
  4. Transplacental Transmission : The diseases like AIDS, German measles and syphilis can also be transmitted from infected mother to the foetus though placenta.
  5. Droplet infection : Pathogens spread by way of sneezing, coughing, spitting and taking as in common cold, influenza, diphtheria, tuberculosis, pneumonia etc.

(ii)indirect transmission : They pathogens of some diseases are carried through some intermediate agents. It occurs in the following ways :

  1. Vectors : They are living organisms which spread their pathogens from an infected person to a healthy person. Usually, a part of life cycle of the pathogen is passed in the body of the vector. Some animals like housefly transfer the pathogen without taking them in their bodies. They are called carriers. Housefly is carrier of cholera, dysentery, typhoid, diarrhoea, etc. Female mosquitoes of many species are vectors of several diseases. They require blood meal in order to obtain nutrients for laying eggs. Female Anopheles spreads malaria while Culex spreads Filaria.
  2. Through contaminated food and water. Cholera, hepatitis B, diarrhoea, ascariasis, etc. are some diseases which are transmitted through contaminated food and water.
  3. Air borne diseases : Infectious agents can get transferred from infected person to healthy person through air, dust and droplets (emitted on sneezing, coughing or spitting), e.g., common cold, pneumonia, tuberculosis.
  4. Fomite borne : Articles coming in contact with patients are a source of infection, e.g., door handles, taps, garments, currency, utensils, crockery.
  5. Pathogenicity : Pathogens can harm their hosts in a number of ways such as by

(i)destruction of body tissues and

(ii)release of toxins or poisons which may be endotoxins. The entry of the pathogen in the body is called infection. After entering into the body, the pathogens multiply till they produce enough toxins to make the symptoms of the disease appear The interval between infection and appearance of first symptom of the disease is called incubation period.


(a)Organ or Tissue Specific Effects :

A microorganisms enters the body through different points like nose, mouth, sex organs etc. Which decides the organ or tissue that micro organism invades. At the same time the signs and symptoms of an infectious diseases also depends upon the tissue being invaded. e.g. If bacteria causing tuberculosis enters through nose, it invades respiratory passage and lungs and its symptoms are cough and breathlessness, but in some cases they may infect other organs also.

 Common Effects :

This category includes effects like inflammation in which swelling, reddening and pain in infected area and increase in body temperature occurs. These effects arise due to the active involvement of immune system to provide defence to body by producing some specific chemicals from WBC’s, against that microbe and this is not confined to a particular organ or tissue but seen in whole body.

(b)Severity of Effects :

It directly depends upon the no. of microorganisms. if microbes are smaller in number their effects are minor and can be overcame by our immune system in a lesser time but if the number of micro organisms inside the body is very high the effects are more severe and long lasting.


The basic concept behind the treatment process is to target the biochemical pathways occurring inside an organism for this certain drugs like antibiotics are prepared to after or stop the biochemical reaction of the microbes at some stage to stop them to produce infections, toxins or to kill them or to check their further growth and multiplication. There are two ways in which these diseases are treated they are :

  1. Reducing the symptoms : By this, infection is not cured but some of the symptoms like fever, pain, aches, inflammation can be reduced to make the patient full comfortable. this is done by medicines like pain killers etc.
  2. killing infectious agents : this can be done by targeting the biochemical pathways of infectious agents using specific drugs.

(a)Drugs :

chemical compounds that targets a particular reaction among the chain of reactions involved in the biochemical pathway by reacting with some substrates of that reaction and resulting in an undesirable product so that reaction cannot proceed further and stop infections and can kill the microbes. they do not affect human cells.

(b)Antibiotics :

Antibiotic are chemicals that kill or stop the growth of certain kinds of microbes. They help our body to fight against diseases. The development of antibiotics began with the discovery of penicillin by sir Alexander Flemming in 1928. Flemming noticed that an agar plate inoculated with bacterium Staphylococcus aureus had become contaminated with a mould. He future noticed the presence of a clear zone in the agar plate in which breakdown of the bacterial cells had occurred. Detailed studies led to the isolation of an inhibitory substance from the mould. As the mould was identified as Pencillium, Flemming called the antibiotic penicillin. Soon other antibiotics were isolated. Some well know antibiotics are streptomycin, gramicidin and tetracycline. the antibiotics have been obtained from either bacteria or fungi.

  1. These are the drugs specific for curing bacterial diseases. they either ceases the formation of cell wall or interferes in their metabolic activities like production of proteins. This kills or stops the growth of bacteria.
  2. Antibiotics are not effective for viruses or it is difficult to make antiviral diseases because Viruses are acellular entities which only have nucleic acid and protein but lacks cytoplasm, cell wall and cell organelles they do not have their own metabolic system but they use the host’s metabolic machinery to grow & multiply so drugs are not effective for them. 


Preventive measures are categorized into two distinct groups:

(a)General preventive measures :

It includes:

  1. Safe drinking water :Drinking water should be filtered to remove suspended particles And boiled, ozonized and treated with chlorine before drinking to avoid water borne diseases like typhoid, cholera, hepatitis etc.
  2. Proper disposal of waste: Garbage should not be dumped here and there rather it should be thrown in covered garbage cans and burnt or buried for disposal. Sewage carrying drains should be covered for proper treatment of diseases of stomach and intestine.
  3. Control of vectors : Growth and breeding of animals like mosquitoes, rats, flies, Cockroaches should be controlled, by keeping surrounding clean, spraying insecticides, removing stagnant water from populated areas.
  4. Strong immune system: It helps to defence our body against invading microbes and can be made strong by proper diet and nourishment.

(a)Immune system: our body possesses a special type of defence mechanism called immune it provides resistance against disease causing microorganisms. immunity is the ability of the body to resist the infections. Two specific types of cells are present in our body that provide immunity.

(b)They are WBC (leucocytes) in blood and lymphocytes in lymph when any foreign body attacks our body these cells are released to all parts of body, they isolate, engulf, kill and digest the infectious agents and thus defend our body against any type of infection.

(b)Specific Preventive Measures :

This can be done two ways :

  1. Immunization: Stimulating the body to produce antibodies by artificial means. our Immune system is misleader, to develop a memory against particular infection by introducing something into the body that mimics the specific microbe. Specific prevention is provided by the immune system. it produces specific molecules called antibodies that fight against the invading microorganism or their products called antigens. Antibodies are pertinacious molecules made by WBC’s and lymphocytes to fight against foreign bodies or other harmful chemicals. Antigens are also proteins or other harmful chemicals that are present on surface of invaders. Whenever there occurs attack of a foreign body specific Antibodies are produced corresponding to that antigen and an antigen antibody reaction occurs. it either engulfs and phagocyte it or makes it harmless and then makes them unable to grow and multiply. Besides  this  immune system also possesses memory. once antibodies are produced they remain in the body and at the second infection they recognize the antigens and show a much faster response. 
  2. Vaccination: A vaccine is a suspension of disease- producing micro-organisms which is Modified by killing or wreaking  (attenuated) so that the suspension will not cause disease. Rather it stimulates the formation of antibodies upon inoculation. The antibodies remain in blood for long and when the germs of a particular disease enter the body, the antibodies destroy them. This is the basis of immunization. 

Some common vaccines :

  1. DPT vaccine, for protection against diphtheria, whooping cough and tetanus
  2. BCG vaccine, for protection against tuberculosis
  3. Polio (OPV) vaccine ÝTyphoid vaccine  ÝMeasles vaccine ÝTT vaccine, against tetanus


  1.  Pulse polio programme: The aim of this programme it to eradicate polio from our country. it was first held in our country in December, 1995. Polio vaccine called Oral polio vaccine (OPV) is given to children orally (through the mouth), as per the National Immunisation Schedule (NIS),NON


These diseases which remain confined to a person. they are neither present at birth nor spread from one person to another. The diseases are caused due to some specific factors. They may be caused due to improper functioning of an organ (short sighted, hypertension, arthritis), hormonal imbalance (diabetes, dwarfism), allergy, cancer, inadequate diet (anaemia, goitre), etc.

  1. These diseases are of following types :
  2. Deficiency diseases : caused due to lack of some nutrient materials in our body like Vitamins, minerals, protein etc.
  3. Degenerative diseases : caused due to ageing or malfunctioning of any organ or part of Body.
  4. Allergies : caused due to hypersensitivity of an organism to certain type of material like Pollen grains, dust etc.
  5. Uncontrolled growth of cells: this can cause cancer and tumor.
  6. Mental disorders
  7. Occupational diseases
  8. Addiction: caused due to excessive intake of drugs tobacco, alcohol etc. 











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