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Transmission and control of infectious diseases
Infectious diseases may be transmitted by direct or indirect methods. Direct transmission
comes about by direct contact with the infected person or by contact with droplets of body fluid.
Diseases transmitted by direct contact are called contagious diseases. Indirect transmission
occurs through an intermediary agent like an insect, air or contaminated water. Carriers of
disease are called vectors. Examples of vectors are the mosquitoes that carry malaria from
person to person.
Natural control
The human body has several mechanisms for coping with disease. The first defense is the outer
layer of the skin, consisting of dead cells. Harmful pathogens can be shed with the dead cells.
There are also lots of good bacteria on your skin, which can help fight invaders. The second
defense is leukocytes (white blood cells), which are able to destroy some pathogens. They travel
to the site of infection via the bloodstream, converge on the pathogens, digest them and engulf
their remains. Dead microorganisms and white cells are left behind. These form the discharge
we call pus.
Our bodies produce special chemicals called antibodies that can protect against some invaders.
A particular antibody will only work specifically against one type of microorganism or toxin. In
a process called acquired (adaptive) immunity, antibodies are produced in response to the
presence of the foreign substance, the antigen, which is why they are so specific in their action.
The antibodies combine with the foreign substance and reduce its effect.
Once the body has produced antibodies against a certain pathogen, it can continue producing
that type of antibody long after the pathogen has left the body. The body will be immune to that
particular pathogen as long as the antibodies are present in the body. But remember, the best
way to protect against disease is to ensure that your immune system is in good shape by eating
well, exercising regularly and having enough sleep.
Inflammation
When you receive an injury, the site tends to swell shortly after. This swelling is called
inflammation and is a natural response to tissue damage. It is a protective response, allowing
increased flow of body fluids to the area to promote healing. However, too much inflammation
means that further damage will be done. This is why ice packs are applied to sports injuries.]
Artificial control
There are now treatments available for many diseases. Of course, the best cure is prevention.
Thanks to the development of vaccines, we are now able to protect ourselves from contracting
some diseases that have killed many people throughout history. If we do become infected,
antibiotics can be used to fight many pathogens, although the treatment of viruses is still
difficult.
Vaccinations
A person can be immunised against a certain disease by being injected (innoculated) with a
vaccine consisting of either living microorganisms, an antiserum that works against the toxins
produced by pathogens, or the toxins themselves. For example, the Sabin polio vaccine uses a
live but non-virulent strain of the disease. Vaccines can be used against both bacterial and viral
diseases.
The two types of immunity that can be produced are active immunity and passive immunity.
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Active immunity is where the body cells are stimulated so that they make their own antibodies.
The vaccine may cause some of the milder symptoms of the disease to appear, but the person
will be protected from a serious attack. Passive immunity, which does not last as long as active
immunity, is achieved by injecting a vaccine that consists of antibodies previously produced by
another organism. This is good in emergency situations, where immunity is needed in a short
space of time.
Even active immunity does not last forever for some diseases. Production of antibodies can
wane with time and a booster shot (reinjection with the vaccine) may be needed. For example, it
is recommended that tetanus booster shots are given every ten years.
Many children are now routinely vaccinated against such diseases as polio, tetanus and measles.
In Australia there have been no cases of polio since 1986 and cases of other illnesses for which
vaccinations are available have been greatly reduced. Some parents choose not to immunise their
children through fear of side effects. In reality, very few children experience any ill-effects and
the benefits far outweigh the small risk involved. It is estimated that for every dollar spent on
immunisation programs, four dollars are saved in public health costs.
The first vaccines
The earliest evidence of vaccinations goes back to around 500 BC. Chinese physicians noted
that exposing healthy people to particles from smallpox scars gave them a milder form of the
disease. This protected them from the more serious form. Only 4% died from this procedure –
not bad for that era.
No great advances were made until 1796. An English physician named Edward Jenner noticed
that milkmaids rarely contracted smallpox. He hypothesised that this was because most had been
infected with a similar, milder disease of cows known as cowpox. Jenner exposed James Phipps,
an 8 year old boy whose family was dying of smallpox, to cowpox and then smallpox. The boy
survived and widespread vaccinations began within a few years.]
Antibiotics
In the late 1940’s Howard Florey discovered that the mold penicillium was effective at killing
certain pathogens. Currently there are a variety of antibiotics available to fight many infections
but they are not effective against viruses. Antibiotics are able to selectively kill off certain
pathogens while leaving the patient’s own body cells intact. They are not effective against
viruses because the viruses are actually inside the body cells.
Unfortunately, overuse and misuse of antibiotics has led to increasing numbers of antibiotic
resistant strains of bacteria. The more antibiotics are used, the more quickly resistant strains
emerge. Given the length of time it takes to develop new drugs (up to 20 years) this is
particularly worrying as we are left without any drugs to fight the new strains. So unless the
infection is severe, it is best for everyone if you let your body recover naturally.
The problem with viruses
Because they are so small, viruses are difficult to isolate in the lab. Another problem with
viruses is that they multiply so rapidly that new strains are appearing all the time. Control of
viral infections has mainly relied on vaccines. With so many different strains, however, it is
difficult to find effective vaccines. There are so many strains of the common cold virus that no
one can ever become immune to all of them. Some chemicals, like AZT which is used for HIV
sufferers, have been shown to be effective in reducing virus effects, but as yet no chemicals can
erradicate a viral infection.
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Questions
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Name one direct and one indirect method for spreading a disease.
Direct: Contact (skin on skin eg.cold sore)
Indirect: Insects eg. Mosquito with malaria
Copy and complete the following sentences using the words below.
infection
immunity
vaccine
innoculated
antibodies
leukocytes
(a)
White blood cells are also called leukocytes.
(b)
An invasion of foreign organisms is called an infection.
(c)
Being inoculated with a vaccine can give a person immunity against certain
diseases.
(d)
A vaccine makes a person’s body produce antibodies.
Give an example of a vector. Insect eg. mosquito
What is pus made up of? Dead microbes and white blood cells
Explain the terms ‘antigen’ and ‘antibody’.
Antigen: foreign substance
Antibody: chemical that is produced by the body and acts against antigens
How does your body know when to produce antibodies?
In response to foreign substances (antigens).
The following passage contains some incorrect facts, spelling and punctuation. Correct
any errors and copy it into your book.
antibodies are part of a group of chemicals called imunnoglobens once your body has
prodused an antibody it can never produse the same one again. Your body is imune as
long as antigens are present
Antibodies are part of a group of chemicals called immunoglobins. Once your body has
produced an antibody it remembers how to make it and keeps producing it. Your body is
immune as long as antibodies are present.
How do the skin and leukocytes protect the body from disease?
Skin is a physical barrier and sheds microbes with dead skin cells. Leukocytes surround
and destroy invading pathogens.
Why are antibiotics ineffective against viruses? Viruses are inside cells and antibiotics
can’t reach them.
Why is the overuse of antibiotics dangerous? Overuse creates antibiotic-resistant
bacteria.
Why are booster shots sometimes necessary? To increase the levels of antibodies.
If you contracted measles as a child you are unlikely to ever get it again? Suggest a
reason for this. No! Because you will have antibodies to it.
Why do so many people get reinfected with the cold virus year after year? Why don’t
they have immunity after one infection? Because it mutates easily and produces new
strains.
Jenner found that infection with cowpox gave immunity against smallpox. Suggest how
an infection with one pathogen could give immunity against a different pathogen.
Cowpox created antibodies that also worked against smallpox.
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