Communicable diseases, also called infectious diseases, are diseases that can spread from person to person. According to the National Institutes of Health (NIH), they are caused by germs, which are small living things found everywhere in the soil, air and water.
Communicable diseases are spread by different means. According to the Mayo Clinic, the means include direct and indirect contact, insect bites and food and water contamination. This fact helps to further classify the diseases and develop strategies for prevention. Another means of classification is by the type of agent causing the disease.
Viral Diseases
Viruses are the smallest in size among the major classes of disease-causing germs. They account for a majority of the worldwide epidemics--widespread cases over a large area--and pandemics--widespread cases involving different continents.
According to the Centers for Disease Control and Prevention (CDC), the influenza virus, which causes the flu, infects 5 to 20 percent of Americans each year, resulting in about 200,000 admissions and 36,000 deaths. The virus is also the cause of the swine flu pandemic that is affecting every continent at the moment. It is typically an airborne infection, although it can be spread by indirect contact via contaminated surfaces and other material. Other important viral illnesses spread the same way include measles, mumps, rubella, chicken pox, poliomyelitis and severe acute respiratory syndrome, or SARS.
The human immunodeficiency virus, or HIV, causes acquired immunodeficiency syndrome, or AIDS. It is a current worldwide killer and is spread by direct sexual contact, by contaminated shared needles or by transfusion of blood and blood products. Other viral illnesses transmitted by close contact include infectious mononucleosis, herpes simplex, human papilloma virus, molluscum contagiosum and hepatitis B and C.
Insects and animals transmit certain viral diseases, including yellow fever, Dengue fever, Colorado tick fever and rabies.
Bacterial Diseases
Bacteria can invade certain organs in the body, producing disease syndromes. Septicemia, the presence of infectious bacteria in the blood stream, is among the top 10 killers in the U.S., according to the CDC. Other severe bacterial infections include tuberculosis, bacterial meningitis, bacterial pneumonia, diphtheria, anthrax, whooping cough, skin sepsis--boils and carbuncles--and streptococcal throat infections. Certain bacteria are spread by food and water contamination. These include cholera, typhoid fever and other forms of bacterial gastroenteritis.
Close personal contact spreads certain bacterial illnesses, like syphilis, yaws, gonorrhea and leprosy.
Fungal Diseases
In normal individuals, fungal infections are typically localized to the skin and mucous membranes, avoiding deeper organs. Fungal illnesses include ringworm, fungal nail infections and other skin infections. Thrush from candida albicans affects the vagina and, rarely, the mouth. Certain other types of fungi cause deeper systemic illness, especially in people with depressed immunity. Such illnesses include aspergillosis, histoplasmosis, cryptococcal meningitis, blastomycosis and coccidiomycosis.
Parasitic Diseases
Parasites can be unicellular, having only one cell unit, or multicellular, with many cells. Illnesses caused by unicellular parasites or protozoa include malaria, toxoplasmosis, trichomoniasis, trypanosomiasis, leishmaniasis, giardiasis, pneumocystis carinii pneumonia, and amebiasis. Illnesses caused by multicellular parasites include helminthiasis, ascariasis, filariasis, enterobiasis, scabies and lice infestation.
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