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Disease is a departure from normal health through structural or functional disorder of the body.

Categories of Diseases

Diseases can be categorised in several ways based on the following factors:

  • extent of occurrence,
  • communicability,
  • type of pathogen and
  • type of transmitting agent.

Diseases Based on the Extent of Occurrence Diseases are classified as follows:

 The disease is found in certain area attacking only a few number of people, e.g. goitre in the sub-Himalayan regions.

: The disease breaks out and spreads from one place to another affecting a large number of people at the same time, e.g. in 1994, the disease plague spread from Surat to many other places in India.

: The disease is distributed worldwide, e.g. AIDS.

: There are scattered individual cases of a disease, e.g. cholera.

Diseases Based on Communicability 
Diseases are classified into two major categories:

  1. Non-communicable or non-infectious: Diseases that are not caused by any germ. These diseases cannot spread from one person to another (non-transmissible). These are caused due to improper functioning of the body.

Types of non-infectious or non-communicable diseases include the following:

  • Metabolic diseases (also called organic diseases): These diseases are caused due to malfunctioning of various body organs. Diabetes is a good example. In this disease, pancreas fails to produce sufficient insulin hormone which controls the sugar level in the blood. Therefore, sugar starts passing out in urine. In case of heart attack orcoronary thrombosis, blood supply to heart muscles is obstructed and cardiac muscles fail to function properly.
  • Genetic diseases or congenital diseases: These diseases develop at the time of embryonic development due to defects in the genes of the chromosomes inherited from parents. Examples include haemophilia, in which the blood clots very slowly due to the absence of certain special factors in the blood.

Thalassemia is a disease of defective haemoglobin of RBC. A child with such defect needs blood transfusion very frequently.

  1. Allergies: These diseases develop due to sensitivity to specific substances in the environment, e.g., hay fever, asthma, etc.
  2. Cancer or uncontrolled growth of body cells: Cancer is an abnormal multiplication of cells. Their uncontrolled divisions may lead to tumour. Cancers are usually fatal. Early detection and treatment improve chances of survival.
    Causes of cancer include chemicals, tars from tobacco smoking, tobacco chewing, drugs, pollution, certain radiations and even certain viruses. Any agent that causes cancer is called a carcinogen.
    Cancer occurs most commonly in those tissues in which cell division is a normal activity. Examples include skin, liver, lining of stomach, uterus, breasts etc.
  3. Degenerative diseases: These diseases are caused due to ageing. With age, deterioration occurs in structure and functioning of body cells and organs. For example,cataract (eye lens turns opaque causing blindness)arthritis (inflammation of joints) and atherosclerosis (hardening of arterial walls reducing blood flow).
  4. Diseases caused due to physical and chemical agents:
  • Sunburn and heat stroke are examples of some diseases caused by physical agents.
  • Chemical agents causing diseases are lead, mercury, potassium cyanide, snake bite etc.
  1. Communicable or infectious diseases: Communicable diseases are transmitted from the source of infection to a susceptible host. Such diseases are caused due to microorganisms such as viruses, bacteria, fungi and protozoa. Following are the modes of spread of diseases:
    1. Diseases through air: Diseases spread through air very easily. A patient suffering from cough and cold emits millions of microbes into the air when he coughs or sneezes, thus contaminating the air. If such an air is inhaled, the infection may spread to a healthy person.
      Tuberculosis, common cold, cough and flu are some of the diseases that spread through air.
    2. Diseases through water: One might have seen people washing clothes, cleaning cattle, urinating near the lake, tanks, river etc. draining of sewage into the river and water bodies. All these pollute water. When such water, contaminated with microbes, is used by human, diseases spread. Cholera, typhoid and amoebic dysentery are some of the diseases which spread through water.
    3. Diseases through animals: The pets or reared animals such as dogs, cows and cats can spread diseases. For example, when milk from a cow suffering from tuberculosis is consumed without boiling, it may cause tuberculosis.
      Dogs and cats may have rabies virus. If the number of virus increases, they suffer from the disease. Bite or licking by an infected animal causes the entry of viruses into our body. A pig may cause brain fever in other animals, and monkeys may spread ‘Kyasanoor forest disease’.
    4. Diseases through insects: Mosquitoes, flies, fleas, cockroaches and bed bugs are the common carriers of disease causing microbes. When we consume food contaminated by these insects or by their bite, microbes gain entry into our bodies. Malaria and plague are some of the diseases spread by insects.
    5. Diseases through soil: The microbes present in soil may enter the body through cuts wounds produced by a rusted knife or blade. For example, while working in the field, microbes may enter the body through wounds produced by equipments.
      Tetanus is one of the diseases that spreads through rusted iron and soil.

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