Infectious diseases are known as communicable

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Infectious diseases
Infectious diseases are known as communicable diseases if they are easily transmitted from person to person. The disease is endemic if small numbers of cases are always present in the population. It only becomes an epidemic if large numbers of people are affected.

Diseases caused by microorganisms

Not all microorganisms are harmful to humans. In fact, some are very helpful. Microorganisms can serve as food sources, decomposers of wastes, help protect us from disease and aid digestion. Only a few microorganisms cause disease. The microorganisms that cause disease are known as pathogens and include certain types of bacteria (singular – bacterium), viruses, protozoa and fungi (singular - fungus).


The yersinia pestis bacterium, formerly called pasteurella pestis, is the pathogen responsible for Bubonic Plague – The Black Death. Between 1347 and 1352, an outbreak of this disease killed a third of Europe’s population – an estimated 25 million people. It was spread by the fleas of rats.

Some examples of pathogenic microorganisms and the diseases they cause.



Disease Caused


Rabies virus
Varicella virus

Vibno cholera

Clostridium botilinum
Giardia lamblia

Toxoplasma gondii

Candida albicans

Tinea capitis




Food poisoning


Ringworm (scalp)

Paralysis, spasms, fever, overproduction of saliva

Fever, itchy blister-like rash

Diarrhea, vomiting and dehydration

Blurred vision, weakness, difficulty swallowing & occasionally death

Nausea, flatulence, diarrhea

Acute form causes fever, chills, rash, exhaustion

Creamy mucous, can be oral or vaginal

Rounded areas of scaling and baldness


Bacteria are present in high numbers all around us. There are about a billion per teaspoonful of soil and there are probably a greater number of bacteria on your skin than there are people on Earth.

One characteristic that is used to identify a bacterium is the shape. They may be rod-shaped (bacilli), spiral (spirilla), or spherical (cocci). All bacteria consist of only one cell, but they can join together in pairs, chains or clusters.

golden staph bacteria (streptococcus aureus)

Bacteria can multiply very quickly under the right conditions. If conditions are not favourable for growth, some types of bacteria can form thick-walled spores that allow them to withstand cold, heat and prolonged drying. They can remain inactive for days or even years.


Viruses are so small that they can only be seen with an extremely high-powered microscope called an electron microscope. Viruses are not considered to be living things because they do not self-reproduce, feed, grow or move. They do move from place to place, but only if something else moves them, like wind or water. Unlike bacteria, viruses are not cells. Because they are not cells, it is much harder to treat a viral infection than a bacterial infection.

Viruses are tiny chemical parasitic invaders that move into cells and take them over. They reprogram the cell to make many more virus particles. Eventually there are so many virus particles inside the cell that it breaks open and the virus particles are free to invade other cells. Some viruses can invade cells and remain inactive for long periods of time. The herpes virus is an example of this. Others do not kill the cell they infect, but reprogram it in a way that causes it to become cancerous.

varicella voster virus

Emerging viruses

New viruses are emerging all the time. One example of this is the Sabia virus, which first showed up in 1990 when a woman in the town of Sabia, Brazil, died from a virus that had been affecting local rodents for years. Another virus, a relative of the common cold known as SMAM-1, is thought to cause obesity – so it may actually be possible to ‘catch’ fatness!


Diseases caused by protozoa are most often seen in tropical and subtropical areas. Like bacteria, protozoa are single-celled. Most protozoa are harmless to humans, but some parasitic types can cause serious illness. Protozoa can form protective cysts around themselves if conditions are unfavourable.

One example of a protozoan infection is the common tropical disease malaria. Malaria is caused by plasmodium, single-celled parasitic protozoa that live in the red blood cells and cells of the liver. Falciparum malaria, the most dangerous type, is fatal in about 20% of untreated cases. Infection initially occurs by a female mosquito bite.

Very few fungi cause disease in humans. The most common types are those that invade the hair, skin and nails. Fungi are opportunistic pathogens. They are not usually associated with infection, but they can cause infection if the conditions are ideal or if the person's immune system is not working properly. AIDS sufferers are prone to opportunistic infections.

Diseases caused by macroscopic parasites

Parasites that can be seen without a microscope are called macroscopic parasites.

The most common type of disease-causing macroscopic parasite is the flatworm. Flukes are the flatworms most well-known for causing disease in many animals, not just humans. Parasitic flukes that affect human beings include intestinal flukes, blood flukes, lung flukes and liver flukes. All of these can cause serious damage to the organs they inhabit and result in chronic illness for the host. For example, blood flukes can damage blood vessels near major organs like the bladder and kidneys.

roundworms (passed from puppy)

Another type of flatworm is the tapeworm, which can sometimes live in human intestines. One tapeworm which affects humans is that which causes hydatids disease. If these tapeworms' eggs are swallowed by humans, the tiny embryos can emerge from the eggs and move from the intestines to the bloodstream. Cysts then develop wherever the embryos ends up. Often this is in the liver. These cysts are capable of killing the host.


1 What is the difference between infectious diseases and communicable diseases?

2 How is an endemic disease different from an epidemic?

3 Name one way that microorganisms can be helpful to humans.

4 What is a pathogen?

5 What is the name of the bacterium that causes Bubonic Plague?

6 What are the symptoms of (a) rabies (b) giardia

7 What are the three common shapes found in bacteria? Draw an example of each.

8 What does the name ‘staphylococcus’ tell us about this arrangement of bacteria? Draw an example.

9 What do viruses do to cells?

10 Why are viruses not considered to be living things?

11 What is an opportunistic pathogen?

12 These sentences all contain errors. Correct them and copy them into your books.

  1. Spherical bacteria are called spirilla.

  2. Viruses are larger than bacteria.

  3. Many fungi cause disease in humans.

  4. Parasites always kill their hosts.

13 How can hydatids disease cause death in humans?

14 How do tapeworms cause malnutrition?

15 Why is it important to describe all your symptoms to your doctor when you are sick?

16 How do bacteria and protozoa protect themselves in unfavourable conditions?

17 Name a tropical region on Earth. Why do you think malaria is more common in tropical regions?

18 How could you protect yourself from malaria when traveling in tropical regions?

19 Explain the difference between macroscopic and microscopic.

20 What precautions could you take to prevent becoming infected with blood flukes?

21 Why are many diseases caused by bacteria but very few diseases are caused by fungi?

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