Viral diseases · Part 2

and the elderly are at risk from severe infection. Amoebiasis also called amoebic dysentery or amoebic colitis is caused by Entamoeba histolytica, which lives in the human large intestine and feeds on mucus and bacteria (Fig. . ) . Infective stage of this parasite is the trophozoite , which penetrates the walls of the host intestine (colon) and secretes histolytic enzymes causing ulceration, bleeding, abdominal pain and stools with excess mucus. Symptoms of amoebiasis can range from diarrhoea to dysentery with blood and mucus in the stool. House flies ( Musca domestica ) acts as a carrier for transmitting the parasite from contaminated faeces and water. Food vacuole Plasmalemma Ectoplasm Endoplasm Endosome Nucleus Fig. . Entamoeba histolytica African sleeping sickness is caused by Trypanosoma species . Trypanosoma is generally transmitted by the blood sucking Tsetse flies. Three species of Trypanosoma cause sleeping sickness in man. . T. gambiense is transmitted by Glossina palpalis (Tsetse fly) and causes Gambian or Central African sleeping sickness (Fig. . ). living cells. Outside the living cells they cannot carry out the characteristics of a living organism. Viruses invade living cells, forcing the cells to create new viruses. The new viruses break out of the cell, killing it and invade other cells in the body, causing diseases in human beings. Rhino viruses cause one of the most infectious human ailment called the “ Common cold ”. Viral diseases are generally grouped into four types on the basis of the symptoms produced in the body organs. i. Pneumotropic diseases (respiratory tract infected by influenza) ii. Dermotropic diseases (skin and subcutaneous tissues affected by chicken pox and measles) iii. Viscerotropic diseases (blood and visceral organs affected by yellow fever and dengue fever) iv. Neurotropic diseases (central nervous system affected by rabies and polio). Some common viral diseases of human beings are given in table . .