ed state of body or mind. We generally associate word disease with physical or mental sickness. The disease can be in any part of the body and may or may not be curable. Most of the time we cure the physical illness. Mental sickness is difficult to be identified as this is a relative term and one takes notice of it only in extreme cases. Being healthy means being free of disease; both physically and mentally. It refers to the state of body and mind which is in harmony with the surroundings or environment. A perfect health is almost difficult to achieve. For time immoreal man has tried to understand the nature and causes of diseases and has tried ways to cure those diseases.Types of Diseases
The diseases can be categorized into two basic types. Congenital, present at birth and acquired that is contracted after birth.
(i) Congenital Diseases:- Congenital diseases are present at the time of birth. Congenital diseases could be due to genetic abnormality or due to malfunctioning of any organ or system. Congenital diseases are permanent, generally not curable and may be inherited to the children.
(ii) Acquired diseases:- The acquired diseases develop after birth and can be broadly classified into two types:
(a) Communicable or infectious diseases
(b) Non communicable or non-infectious diseases. These diseases can further be divided into many types.
The agents that cause the diseases can be put in the following five categories.
(i) Biological agents or pathogens like viruses, bacteria, fungi, protozoans, etc.
(ii) Nutrient agents like minerals, carbohydrates, proteins, fats and vitamins.
(iii) Chemical agents like urea or uric acid formed in the body; or pollutants, spores and pollens present in the atmosphere.
(iv) Physical agents like heat, cold, humidity, pressure, radiation, etc.
(v) Mechanical agents like friction or other mechanical forces.
Diseases caused by Pathogens
Pathogens are disease causing living organisms like bacteria, yeasts, molds, protozoans and parasitic worms. The first study as to how they cause diseases came from the work of Robert Koch, a German physician. In 1876 he showed that the disease anthrax of sheep was due to bacteria Bacillus anthracis which existed in the pastures in the form of protective spores. He found these rod shaped bacteria in the blood vessels of infected sheep also and so came to a conclusion that sheep and cattle got these bacteria while grazing in the pastures. He isolated and grew the bacteria in the lab and inoculated them into healthy mice. The mice developed anthrax and died. On the basis of his findings, Robert Koch – the father of bacteriological technique postulated that a bacteria (microorganism) causing a disease must satisfy the following four conditions.
(i) It must found in the organisms that are infected with the disease.
(ii) One should be able to isolate it and grow it in the pure culture in the lab on the artificial medium.
(iii) When a healthy organism is inoculated with the cultured microorganism, it must contract the disease with the same symptoms.
(iv) The same microorganism should be present in huge number in the inoculated organism. It could be re-isolated from this organism
The conditions given above hold good for animal diseases as well as human diseases. They do not apply to viruses as they cannot be cultured on the artificial medium. Viruses need a living cell to multiply. The outstanding work of Koch and later of Louis Pasteur brought many infectious diseases under control.
Various ways of transmission of infectious diseases
The infectious diseases can be transmitted by various means and if not controlled can take an epidemic form and can cause many diseases. The reason for it is that each pathogen which causes the disease thrives well and multiplies in a specific medium where it does not cause the disease. Such a medium is called as reservoir of infection. The reservoir for human diseases could be other people, animals, soil or water. From these reservoirs, the diseases can be transmitted to healthy persons directly or indirectly.
(A) Direct Transmission :- The pathogens reach and infect a healthy person directly without an intermediate agent. It could be by various means like:
a. Direct contact between the infected and the healthy person. It includes diseases like small pox, chicken pox, syphilis, gonorrhea, etc. Direct contact with the discharge from the lesions, sores, skin or mucous membranes causes the infection.
b. Droplet infection: A tiny droplet is sufficient to cause an infection or a disease. Most air-borne respiratory infections like cold, influenza, whooping cough, pneumonia and tuberculosis occur in this way. The droplets emitted into the air from sneeze, cough, spit or while talking are laden with bacteria and viruses. By inhaling the air containing the droplets one gets the infection.
c. Contact with the soil containing disease causing bacteria, viruses
and other organisms. Tetanus causing bacteria thrive well in soil and enter the organism through wound or injury.d. Bite of an animal: Rabies or hydrophobia in man is caused by a virus that is transmitted through a dog bite. The virus is present in the saliva of the rapid animal and enters the healthy person through the wound.
e. Transplacental transmission: Pathogens of certain bacteria like German measles and syphilis can pass through the placenta and if mother has these diseases during pregnancy, the foetus can also contract them.
(B) Indirect Transmission :- Many times the transmission of a pathogen to a healthy person is not direct but through an intermediate agent.
a. By vectors: It refers to arthropod carriers like houseflies, cockroaches, female anopheles mosquito, etc. which carry infectious organisms and spread the diseases. Malaria is contracted by the transfer of pathogen plasmodium by the female anopheles mosquito.
b. Vehicle borne: The pathogens present in agencies like food, water, ice or blood generally enter the body through mouth. They normally infect the digestive system and hence the wastes excreted are highly infectious. The diseases transmitted through these include cholera, dysentery, etc.
c. Air borne pathogens spread by wind currents, aerosol sprays and dust. Air and dust contain number of bacterial spores that can infect organisms.
d. Fomite-borne means one gets infection by touching any article that has been in contact with the infection. The articles could be garments, toys, crockery, door handles, soaps, taps, surgical instruments and syringes.
e. Unclean hands and fingers are the source of infection for various digestive and respiratory diseases.
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