The following points highlight the two major types of diseases that occur in humans. The types are: 1. Communicable or Infectious Diseases 2. Non-Communicable or Non-Infectious Diseases.
Type # 1. Communicable or Infectious Diseases:
These can be transmitted from an infected person to a healthy person by means of air, water, food, physical contact or vectors. These are caused due to infection and multiplication of some kind of micro-organisms, so are also called infectious diseases.
Classification of communicable diseases. These can be categorized on two basis:
(i) Causative Agent, and
(ii) Mode of Transmission.
(i) Depending upon the causative agent, communicable diseases are of six types:
1. Bacterial diseases e.g., diphtheria, whooping cough, leprosy, syphilis, tetanus, typhoid, plague, pneumonia, tuberculosis, cholera, anthrax, etc.
2. Viral diseases e.g., dengue, influenza, measles, polio, smallpox, chickenpox, common cold, rabies, Japanese encephalitis, AIDS, infectious hepatitis, etc.
3. Protozoan diseases e.g., malaria, amoebiasis, kala azar, sleeping sickness etc.
4. Helminth diseases e.g., taeniasis, ascariasis, elephantiasis, trichinosis, liverrot, echinococcosis, etc.
5. Fungal diseases e.g., ring worm, athlete’s foot, candidacies, etc.
6. Rickettsial diseases e.g., typhus fever, trench fever, Q-fever, Rocky mountain spotted fever etc.
(ii) On the basis of their mode of transmission, the communicable diseases are of two types:
a. Contagious Diseases:
These communicable diseases can spread from an infected person to healthy person by actual contact between them e.g., STDs, smallpox, chickenpox, measles, leprosy etc.
b. Non-Contagious Diseases:
These can spread from an infected person to healthy person with food, air or water e.g., taeniasis, ascariasis, cholera, tuberculosis, typhoid etc., or micro-organisms are injected inside the human body by some carrier or vector hosts e.g., malaria, filariasis, plague etc.
Type # 2. Non-Communicable or Non-Infectious Diseases:
These do not spread from an infected person to a healthy person.
These are of four types on the basis of their causative agents:
(i) Deficiency Diseases:
These occur either due to deficiency of some nutrients in the diet or some hormone e.g., kwashiorkor (protein), diabetes mellitus (insulin), dwarfism (growth hormone), etc.
(ii) Degenerative Diseases:
These occur due to degeneration of certain body tissues e.g., cardiovascular diseases (of heart and blood vessels), stroke disease (of brain) and arthritis (of joints).
(iii) Cancerous Diseases:
These occur due to uncontrolled growth and division of cells in certain body tissues leading to tumour formation.
(iv) Allergic Diseases:
These occur due to hypersensitivity of body to certain external agents, called allergens, e.g., asthma, hay fever, etc.