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The previous chapter mentioned that different types of microorganisms interact with human bodies on a regular basis. They can be harmless, harmful or beneficial. Harmful microorganims are also called pathogenic. This chapter looks at disease- causing bacteria and viruses.

Communicable diseases

The ability of a microorganism to cause disease is called pathogenicity. There are several pathogens that can cause serious harm or even immediate death.

Invasion and multiplication of pathogenic microorganisms in the body is called an infection. When we are infected by pathogens we become sick, which means that our bodies stop functioning properly. Infectious agents, such as bacteria, a virus, fungi or protozoa cause communicable diseases. Communicable diseases can be spread from one person to another.

Infection transmission

All living organisms have a natural or acquired resistance mechanism called immunity. When we get sick, for example, we use different body cells and chemicals to fight bacteria. Bacteria in their turn use different chemicals to fight us. That is why infection is sometimes referred to as a race between pathogen and host organism. The infection can be transmitted by direct or indirect contact.

Direct contact transmission - involves any direct contact with an infected individual. Infection can be passed in water droplets through a sneeze, cough, laugh or exhalation and through bodily fluids. Most communicable diseases like colds, influenza, tuberculosis and HIV are spread from person to person through infected fluid droplets.

Indirect contact transmission - is a method of spreading infection from person to person that involves contact with a contaminated object. Objects can become contaminated when touched by someone with an infection. The infected object is called a fomite.

Another form of infection transmission through indirect contact is through the oral-feacal route, which usually involves ingesting contaminated water.

A third method of indirect contact involves vector-borne diseases, which are carried by animals and insects. A vector is an organism that serves as a 'go-between' in the transmission of a host-to-host disease.

Examples of diseases that can be transmitted via indirect contact are cholera, Salmonellosis and dysentery. See image 1.

Types of pathogens

See image 2.
Natural or human-triggered changes in the environment might upset the natural balance between living organisms. These new environmental conditions may encourage pathogens, allowing them to multiply rapidly and increase the risk of exposing humans who share that environment. Here are the main groups of human pathogens with some examples of the diseases they cause.


E.coli causes food poisoning and urinary tract infections.
Mycobacterium tuberculosis causes tuberculosis.


Influenza virus causes 'flu'.
Herpes simplex virus causes herpes.


Plasmodium causes malaria.


Tinea causes ringworm.

See animation 1.


Epidemiology is the study of the patterns, causes and prevention of human diseases. An outbreak of a contagious disease that spreads rapidly and widely is called an epidemic. An epidemic occurring over a very large area is called a pandemic. The term 'epidemiologic triangle' is used to describe the intersection of host, agent and environment in analysing an outbreak.

The prevalence of a disease is the number of diseased individuals at any one time (point prevalence) or over a given period (period prevalence). The incidence is the number of new cases of a disease that occur within a defined population over an established period of time.

History of epidemics

The influenza (virus-caused) pandemic of 1918-1919 killed more people than World War I. It is considered the most devastating epidemic in recorded world history. More people died of influenza in a single year than in four years of the Black Death Bubonic Plague from 1347 to 1351.

Bubonic plague is caused by bacteria called Yersinia pestis. These bacteria live in fleas on rats. Rats were abundant in medieval times due to very low hygiene standards, and the lack of sewage systems and hot, running water. Bubonic plague killed about a quarter of the European population of that time. Today this disease can be controlled by antibiotics but outbreaks sill occur sometimes. See image 3.

Prevention of infections

The best way of fighting disease is to prevent it. The discovery and manufacture of antibiotics and vaccines in the early part of the 20th century have changed the state of people's health forever. There are three stages of prevention of infections: primary, secondary and tertiary.

The Primary stage - involves public education about infectious diseases.

The Secondary stage - involves treating the actual infection that has already occurred by quarantining and/or vaccinating of infected individuals.

The Tertiary stage - involves the recovery from illness.


Bioterrorism is the deliberate use of microorganisms, or their toxins, to cause death or disease. Biological and chemical agents that could be used include anthrax, small pox, West Nile virus and cholera. Biosecurity refers to the policies and measures taken for protecting a nation's food supply and agricultural resources from both accidental contamination and deliberate attacks of bioterrorism.

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