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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).
Plague
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.
Agent
Type
Disease Caused
Symptoms
Rabies virus
Virus
Rabies
Paralysis, spasms, fever,
overproduction of saliva
Varicella virus
Virus
Chickenpox
Fever, itchy blister-like
rash
Vibno cholera
Bacteria
Cholera
Diarrhea, vomiting and
dehydration
Clostridium
Bacteria
Food poisoning
Blurred vision, weakness,
botilinum
difficulty swallowing &
occasionally death
Giardia lamblia
Protozoa
Giardia
Nausea, flatulence,
diarrhea
Toxoplasma gondii
Protozoa
Toxoplasmosis
Acute form causes fever,
chills, rash, exhaustion
Candida albicans
Fungi
Thrush
Creamy mucous, can be
oral or vaginal
Tinea capitis
Fungi
Ringworm (scalp) Rounded areas of scaling
and baldness
Bacteria
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.
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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
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!
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Protozoa
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.
Fungi
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.
Questions
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What is the difference between infectious diseases and communicable diseases?
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How is an endemic disease different from an epidemic?
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Name one way that microorganisms can be helpful to humans.
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What is a pathogen?
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What is the name of the bacterium that causes Bubonic Plague?
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What are the symptoms of (a) rabies (b) giardia
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What are the three common shapes found in bacteria? Draw an example of each.
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What does the name ‘staphylococcus’ tell us about this arrangement of bacteria? Draw
an example.
What do viruses do to cells?
Why are viruses not considered to be living things?
What is an opportunistic pathogen?
These sentences all contain errors. Correct them and copy them into your books.
(a) Spherical bacteria are called spirilla.
(b) Viruses are larger than bacteria.
(c) Many fungi cause disease in humans.
(d) Parasites always kill their hosts.
How can hydatids disease cause death in humans?
How do tapeworms cause malnutrition?
Why is it important to describe all your symptoms to your doctor when you are sick?
How do bacteria and protozoa protect themselves in unfavourable conditions?
Name a tropical region on Earth. Why do you think malaria is more common in tropical
regions?
How could you protect yourself from malaria when traveling in tropical regions?
Explain the difference between macroscopic and microscopic.
What precautions could you take to prevent becoming infected with blood flukes?
Why are many diseases caused by bacteria but very few diseases are caused by fungi?
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