Diseases can be classified as genetic, metabolic, or infectious. Genetic diseases are caused by genetic defects inherited from the parents. Sickle cell anemia and cystic fibrosis are two examples of genetic diseases. Metabolic diseases are those that may develop from the failure of normal bodily functions, but may also be inherited. Diabetes mellitus, for example, is a metabolic disease characterized by high blood sugar level resulting from insufficient insulin secretion by the pancreas. Obesity is a major contributing factor to adult-onset diabetes. Infectious diseases or communicable diseases are caused by bacteria, viruses, and parasites that use our body as a host for reproduction. Tuberculosis, malaria, and AIDS are responsible for approximately half of all deaths caused by infectious diseases worldwide.
E. Coli bacteria
Hygiene is the science that deals with the promotion and preservation of health by reducing harmful levels of germs through cleanliness and sterilization. The two most common hygienic practices are: 1) washing hands and food preparation areas with soap, and 2) cooking food and boiling drinking water. Washing with soap removes oils and breaks up dirt particles so they may be washed away, whereas cooking and boiling kill harmful organisms that cannot be removed by washing. You can prevent diseases caused by viruses, bacteria, and parasites by keeping a clean environment and by handling food in a sanitary manner. Most intestinal parasites are transmitted by contact with feces from an infected person or pet. These are some of the most important sanitation practices to help you maintain your health:
Viruses penetrate the cell walls of body tissues bringing a payload of DNA or RNA that starts replicating the virus inside the infected cells. When the cell dies, multiple copies of the virus are released and continue to infect other cells. The human body sometimes is able to build antibodies that prevent the replication of the virus to stop the infection. Vaccines consist of weakened viruses that stimulate the body's defenses to fight infection by the natural viruses. Here is a list of diseases caused by viruses:
Bacteria are one-celled organisms that do not have membranes binding their nuclear material (prokaryotes). This feature distinguishes them from protozoa which have a more complex cellular structure and a distinct nucleus (eukaryotes). Not all bacteria cause diseases. Bacteria are present in some fermented foods. Yogurt, for example, has Lactobacillus bulgaricus and Streptococcus thermophilus bacteria. The human mouth and intestines harbor over 400 different types of bacteria that produce some vitamins and ferment fiber to produce short-chain fatty acids. Bacteria that cause diseases are called "pathogenic" bacteria. Antibiotics, like penicillin, are substances that are toxic to the bacteria, but relatively harmless to people. They have been very useful in combating disease, but their increased use has produced antibiotic-resistant bacteria. Bacteria are classified by their shape and the color that they take when stained. A bacillus is elongated like a sausage, a coccus is round, and a spirochete is helical (spiral) in shape. Hans Christian Gram (1853-1938) developed a staining technique that colors some bacteria deep blue (Gram positive), whereas others take on a red color (Gram negative). Here is a list of common bacteria and some of their attributes:
Parasites may be protozoa, fungi, or multi-cellular organisms. Many parasites have complex life cycles that insure their proliferation and survival.
As mentioned above, protozoa have a more complex organization than bacteria. Some protozoa form cysts that protect them from harsh conditions and enable them to live outside of a host for a long time. Some diseases caused by protozoans are:
Malaria, a debilitating disease that causes high fevers, is contracted through the bite of an infected mosquito. The parasite reproduces within the human liver and red blood cells (shown above), and it is transmitted back to other mosquitos when they feed on the infected person. Mosquitoes and flies are also vectors of parasitic diseases such as sleeping sickness and elephantiasis.
Fungi include one-celled yeasts slightly bigger than bacteria, and multi-celled mushrooms and molds. Fungi do not have chlorophyll to make their own food, so they get their nutrition as parasites or by breaking down remains of dead plants or animals. Some fungi are poisonous (e.g., Amanita mushrooms), but some have beneficial uses. For example, Penicillium notatum produces the antibiotic penicillin and Saccharomyces cerevisiae is the yeast used to make bread rise and to brew beer. Fungal diseases are called mycoses and include:
Some fungal infections are more common in certain geographic areas.
Blastomycosis is caused by a fungus (Blastomyces dermatitidis) typically found in decomposing organic material like mulch. The fungus is common in the Great Lakes area of the Upper Midwest and the southeastern and south central regions of the United States. The spores are released into the air when soil is disturbed by farming, construction or wind.
Coccidioidomycosis, also known as valley fever, is caused by fungi (Coccidioides immitis or Coccidioides posadasii) found in the desert soils of the Southwest, most notably in California and Arizona. These fungi also can be released into the air by wind or road traffic that disrupts the soil.
Histoplasmosis is a lung infection caused by the fungus Histoplasma capsulatum, which is commonly found in the Midwest along the valleys of the Mississippi, Missouri and Ohio rivers. Fungal spores may also occur in bird and bat droppings found in farms, abandoned buildings, caves and bird roosts.
Recommended prevention measures can include use of gloves, facemasks and respirators for persons engaged in agriculture, construction and other activities that generate dust or require handling soil.
Multicellular parasites include various kinds of worms and fungi that cause diseases such as:
Intestinal parasites are commonly transmitted by eating food contaminated with feces or by eating raw or undercooked meat and seafood. In many parts of the world, manure or feces are used for fertilizing crops, and this is how vegetables and drinking water can become contaminated with the eggs or larvae of parasitic organisms. It is estimated that the human pinworm affects approximately 50% of the children in the United States. Pinworms lay their eggs around the anus and cause itching. The natural instinct to scratch, in combination with habits like thumb sucking and poor hand-washing, spreads the parasites. Tapeworms and roundworms can infect the body when their eggs or larvae are swallowed by eating raw or undercooked meats (pork) or seafood (ceviche, salmon roe, sashimi, sushi, cold-smoked fish). Protozoan parasites are frequently present in raw oysters.
Hookworms (Necator and Ancylostoma) are generally contracted by walking barefoot on soil contaminated by feces from infected animals or persons, or by swimming or wading in contaminated water. Hookworm larvae are capable of penetrating the skin in a few seconds. Once inside the body, they migrate to the lungs via the capillaries, and then cross into the air sacs of the lungs. They are eventually coughed up and swallowed. In the intestines, they feed on blood and reproduce. The females lay eggs that are passed in the feces to repeat the cycle.
Fungi and molds cause a variety of skin diseases such as athlete's foot, jock itch, and ringworm, but they can also cause pulmonary and internal infections such as aspergillosis, and candidiasis (yeast infection). The dermatophyte Trichophyton rubrum is a fungus that frequently causes chronic infections of the skin and nails. Tinea capitis is a fungal infection of the scalp that causes severe itching, dandruff, and bald patches. Tinea corporis, commonly known as ringworm, is an infection of the skin found on the body, such as the trunk and limbs. Tinea pedis is a superficial fungal infection affecting the feet, often called athlete's foot. This type of fungal infection typically is found in the skin between the toes, and is characterized by itching, blisters, cracking, and scaling. Tinea cruris, also known as jock itch, is a fungal infection of the skin of the groin area. Fungal infections are aggravated by sweating, restrictive garments, not washing and drying the body carefully on a daily basis, and by not changing into clean clothing frequently enough.
Ringworm and other fungus skin infections can be treated with a variety of non-prescription antifungal ointments, powders, or sprays containing tolnaftate, clotrimazole, undecylenic acid, terbinafine hydrochloride, salicylic acid, etc. White vinegar (5% acetic acid) is a common home remedy for fungal skin infections such as ringworm and athlete's foot; it is applied with a cotton ball in the morning and in the evening, but it produces a burning sensation on broken skin. To prevent re-infection, the fungi that cause skin infections should be eliminated from environmental surfaces like bathtubs and toilet seats by washing the surfaces with diluted household chlorine bleach, which is a solution containing typically 5.25% sodium hypochlorite (NaOCl). Fungi on towels, sheets, socks, underwear, and other clothing can also be killed by adding bleach during the wash cycle, although this may fade some fabrics. In the case of athlete's foot, shoes should also be treated with antifungal foot powder. For tinea capitis infections, pillows, pillow cases, combs, hats, and hair brushes must be disinfected. Chlorine bleach should not be used on the skin because it is extremely caustic and will cause chemical burns. Wear rubber gloves when handling bleach.
The Center for Disease Control has a comprehensive database about parasites.
Virus picture courtesy of Stanford Synchrotron Radiation Laboratory.
Escherichia coli picture courtesy of the University of Florida
Cutaneous larva migrans lesions from hookwoorm © Gary Williams, MD
Digitate human papillomavirus wart © Gary Williams, MD