Viruses are simple, a cellular entities consisting of one or more molecule of either DNA or RNA enclosed in a coat of protein. More than 400 different viruses can infect humans. Some viruses can be transmitted through the air and directly or indirectly involve the respiratory system. Many also produce systemic disease. Most of these viruses are highly communicable and cause diseases such as chicken pox, influenza, measles, mumps, respiratory syndromes and viral pneumonia. Some viruses are transmitted from host to host by direct contact and cause diseases such as AIDS, cytomegaloviruses inclusion disease, genital herpes, rabies and serum hepatitis.
Bacteria are small and very simple in structure and have a plasma membrane. The cytoplasmic matrix typically contains several constituents that are not membrane-enclosed. Most bacteria can be divided into gram-positive and gram-negative forms on the basis of their cell wall structure and response to gram stain. Components like capsule and fimbraie are located outside the cell-wall. Of all the known bacterial species, only a very small number are pathogenic to humans. Most of the airborne diseases caused by bacteria involving the respiratory system. Examples include diptheria, Streptococcal disease, tuberculosis and whooping cough. Most of the direct bacterial diseases involve the skin or underlying tissues. Example includes anthrax, leprosy, syphilis. The feed-borne and water borne bacterial diseases are contracted when contaminated food or water is ingested.
Fungi are widely distributed and are found wherever moisture is present. Fungi exist primarily in the form of filamentous hyphae. A mass of hyphae is called mycelium. Although thousands of species of fungi are found in the environment, only 50 are capable of producing diseases in humans. The fungal diseases known as mycoses (Greek mykes, fungus), are divided into five groups according to the type of the infected tissue in the host: superficial, cutaneous, subcutaneous, systemic and opportunistic mycoses. The opportunistic mycoses can create life-threating situations in the compromised host. Examples of those include aspergillosis, and candidiasis.
Protozoa are animal like protists exhibiting heterophilic nutrition and various types of locomotory ability. About 20 different protozoa cause human diseases that afflict hundreds of millions of people throughout the world. Examples include amebiasis, giardiasis, malaria, pneumocyatis pneumonia, the hemoflagellate diseases, toxoplasmosis and trichomoniasis. There are over 150 million cases of malaria, at least 8 million cases of trypanosomiasis, 12 million cases of leishmaniasis and over 500 million cases of amebiasis in the world each year