Infective Disorders

Here the disease is caused by invasion of the body tissues by a living parasitic organism. This organism may be of unbelievably small size, which may not even be seen by an ordinary microscope, e.g. a virus, or even a multi-cellular parasite like an intestinal worm. Between these two, stretches a vast number of organisms- Bacteria, Spirochetes, Vibrios, Rickettsia, Fungi and protozoa. All these have the property of invading the host and producing harmful effects on host cells or depriving nutrition of the host tissues or producing some toxic substances harmful to the body. Common examples of this group of diseases are tuberculosis, pneumonia, typhoid, cholera, amoebic infections, malaria, smallpox, infective hepatits, etc. Sometimes an infective disease suddenly spreads in communities on a wide-spread scale. This is called an epidemics are governed by a large number of factors including the virulence of organisms, chances of spread of germs, community resistance (immunity) and so on. The discipline dealing with this subject is called epidemiology. Control of epidemic diseases is one of the most important and praiseworthy achievements of advanced countries of the west and America. Their higher longevity as compared with ours is directly attributable to this factor. More details about these diseases follow in succeeding chapters.