Infectious Organisms

Bacteria are microorganisms that take on different shapes. Depending on their shape, bacteria can be classified into three types: cocci (spherical), bacilli (rod-shaped), and spirochetes (spiral-shaped). There are bacteria that are bad, called pathogenic, as well as good bacteria.

Microscopically small infectious agents, viruses can only replicate inside living cells. Viruses cannot replicate on their own. For viruses to replicate, they must infect cells and utilize the components of the host cell.

Protozoa are microscopic organisms of one cell that can exist free-living or in parasitic forms. Their ability to multiply in humans is a factor in their survival and permits serious infections from just one organism. Infections caused by protozoa include amoebiasis, trichomoniasis, and malaria.

Generally, all helminths can be seen with the naked eye in their adult stage as they are large, multicellular organisms. The helminth can be both free-living and parasitic, just like protozoa. In humans, there are three main types of helminths: 1) Flatworm 2) Thorny-headed worm 3) Roundworm.

Bacteria or archaeons with a round, spherical, or ovoid shape are called cocci (plural cocci). A coccus is the shape of the bacteria and can have several genera, such as staphylococci or streptococci.

This rod-shaped bacterium is known as a bacillus (plural: bacilli). Among the common diseases caused by these bacteria are typhoid, dysentery, salmonellosis, tuberculosis, botulism, and tetanus.

Vibrio, a Gram-negative bacteria with a curved rod (comma) shape, can cause foodborne illness when eaten undercooked. A common disease caused by these bacteria is cholera and gastroenteritis.

Spirochetes are twisted bacteria that move in a corkscrew-like manner. These bacteria cause diseases such as Lyme disease and syphilis.

Rickettsias are gram-negative bacteria of the genus Rickettsia, carried as parasites by ticks, fleas, and lice, which cause typhus and other diseases.

Chlamydia trachomatis, commonly known as chlamydia, causes the greatest number of blindness cases and is the chief cause of sexually transmitted diseases. The primary symptoms in males are urethritis and in females, endocervicitis. An untreated chlamydial infection in men can cause epididymitis and proctitis.