Types of Pathogens: Bacteria, Viruses, Fungi, and Protozoa

Introduction to Pathogens

Pathogens are disease-causing or infectious microorganisms (EPA 2011, Kennedy 2012). Some are commonly found in water from sewage discharges, leaking septic tanks, or runoff from feedlots. They enter the body and cause disease every day through the air we breathe, food, water, or direct personal contact. The body's immune system is able to destroy many pathogens. When it cannot, infection occurs and the person becomes sick. There are different types of pathogens, but the most common are viruses, bacteria, fungi, and protozoa (EPA, Kennedy).

Most Common Types and Their Differences

Bacteria are single-celled microorganisms that have no nucleus (DHSS 2012, Kennedy 2012). They cause many diseases, a number of which can be prevented by vaccination. Bacteria do not usually kill the infected person but instead weaken the host significantly. These non-fatal bacteria do not spread easily. Examples of bacteria include anthrax, Escherichia coli, salmonella, the streptococcus group, the staphylococcal group, tetanus, gonorrhea, and diphtheria (DHSS, Kennedy).

A virus is smaller than a bacterium and exists as a cross between a living and a non-living organism (DHSS 2012, Kennedy 2012). It has an RNA or DNA core and an outer protein coat. It attacks the cells of the host, where it multiplies rapidly by using the components of the host's own cells. The viral matter destroys and breaks open the cell, and the new copies proceed to infect other cells. A virus can grow only inside living cells. Examples of viruses include those causing smallpox, the common cold, influenza, herpes, and HIV (DHSS, Kennedy).

A fungus is a non-motile microorganism with filaments that requires organic nutrition for survival (DHSS 2012, Kennedy 2012). Most fungi thrive on dead organic matter. Most are considered opportunistic in that they attack when the host body is weakened by another infection, has an open wound, or has AIDS. This type of pathogen attacks the skin or hair and causes infections that are difficult to treat. Fungi are dangerous or fatal when they attack vital organs and the nervous system. Examples of fungal infections include ringworm, jock itch, candidiasis, and histoplasmosis (DHSS, Kennedy).

Transmission and Development of Infection

Protozoa are single-celled organisms with defined nuclei (DHSS 2012). Many of them are human parasites, as in the case of malaria, giardiasis, and toxoplasmosis (DHSS).

Pathogens can enter the body through any opening (Kivi 2010). Entry points include cuts in the skin, the lungs, the mouth and digestive system, mucous membranes, the eyes, and the ears. Pathogens can enter the body if the skin is dirty, broken, or cut. This is why doctors and nurses wear gloves and use only sterile needles and medical instruments in their practice. Some pathogens are airborne and can be inhaled. The healthy body produces nasal mucus that traps these particles and expels them. However, some pathogens are able to withstand the mucous barrier and enter the lungs. Pathogens can also enter the body through the mouth and digestive system by way of infected foods or drinks. Touching the mouth, biting, and licking can also introduce airborne pathogens into the mouth. They may additionally enter through the sexual organs, the anus, the eyes, or the ears. Any contaminated object placed in contact with these openings can transmit pathogens (Kivi).

The development of an infection begins when a pathogen attaches to the cells of the host's body (Tunkel 2008). Many pathogens produce toxins or poisons — for example, Clostridium tetani produces toxins that cause tetanus. The pathogen multiplies in order to spread the infection. In doing so, it either defeats the body's natural defenses, causes a chronic infection, or is destroyed and expelled by the body on its own or through treatment. Some pathogens block the body's natural defenses by interfering with the production of natural antibodies, enclosing themselves in protective outer coats, resisting destruction by substances in the bloodstream, or producing substances that deflect the action of antibiotics. Some pathogens also develop resistance to particular drugs (Tunkel).