B1c: Staying healthy

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Diseases

How are infectious diseases caused?

Infectious diseases are caused by pathogens. Pathogens are microorganisms that cause disease. There are 4 types:

fungi - eg. athlete's foot

bacteria - eg. cholera

viruses - eg. flu

protozoa (single-celled organisms) - eg. dysentery

The symptoms of an infectious disease are caused by cell damage or by toxins produced by pathogens.

What are non-infectious diseases?

Some diseases are not caused by pathogens and so are not infectious. For example:

scurvy is caused by vitamin C deficiency

anaemia is caused by iron deficiency

diabetes and cancer are disorders of the body.

Some disorders are genetically inherited, such as red-green colour vision deficiency.

Example: Malaria

Malaria is caused by a protozoan. Its carried by mosquitoes.

The protozoan is a parasite - an organism that lives off another organism (called a host) and often causes it harm.

The mosquitoes are vectors, meaning they carry the disease without getting it themselves. They pick up the malaria parasite when they feed on an infected animal. Every time the mosquito feeds on another animal it infects it by inserting the parasite into the animal's blood vessels.

To reduce the spread of malaria, it is necessary to reduce the amount of mosquitoes by:

The areas of water where mosquitoes lay their eggs in can be drained or sprayed with insecticides.

Fish can be introduced to eat mosquito larvae

People can be protected from mosquitoes using insecticides and mosquito nets.

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Defending Pathogens

How does the human body defend itself against pathogens?

skin provides a barrier - dead outer cells are difficult for microbes to penetrate, and

the sebaceous glands produce oils that help kill microbes.

blood clotting prevents entry of pathogens

pathogens are trapped by mucus in airways

hydrochloric acid in the stomach kills pathogens.

How does the immune system deals with pathogens?

Once pathogens have entered the body they'll reproduce rapidly unless they're destroyed.

That's the job of the immune system, and white blood cells are the most important part of it.

White blood cells travel around in blood, constantly patrolling for pathogens. When they come across an invading microorganism they have 3 lines of attack:

Consuming - White blood cells can engulf foreign cells and digest them.

Producing Antitoxins - Antitoxins counter the effect of any poisons (toxins) produced by the invading pathogens.

Producing Antibodies -

Every pathogen has unique molecules on the surface of its cells - no two pathogens have the same ones. These molecules are called antigens.

When your white blood cells come across a foreign antigen (like those on the surface of a bacterium) they‘ll start to produce proteins called antibodies, which lock on to and kill the new invading cells. The antibodies produced are specific to that pathogen - they won't lock on to other pathogens.

Antibodies are then produced rapidly and flow all round the body to kill all similar bacteria or viruses.

Some white blood cells stay in the blood after the pathogen has been fought off - these are called memory cells. If the person is infected with the same pathogen again these cells will remember it and immediately make antibodies to kill it - the person is naturally immune to that pathogen.

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Immunisation

What is immunisation?

When you're infected with a new pathogen it can take your white blood cells a while to produce the antibodies to deal with it. In that time you can get very ill, or maybe even die.

To avoid this you can be immunised (vaccinated) against some diseases. eg. polio or measles.

Immunisation involves injecting dead or inactive pathogens into the body. These carry antigens, so even though they're harmless they still trigger an immune response - your white blood cells produce antibodies to attack them.

Some of these white blood cells will remain in the blood as memory cells so if live pathogens of the same type ever appear, the antibodies to kill them will be produced immediately.

What immunity is immunisation classed as?

Immunisation is classed as active immunity.

What are the different types of immunity?

Active immunity is where the immune system makes its own antibodies after being stimulated by a pathogen. It includes becoming naturally immune and artificially immune (immunisation). Active immunity is usually permanent.

Passive immunity is where you use antibodies made by another organism, eg. antibodies are passed from mother to baby through breast milk. Passive immunity is only temporary.

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Benefits and Risks of Immunisation

Advantages:

Immunisation stops you from getting ill.

And if most people are immunised the disease won't be able to spread as easily.

Disadvantages:

But there can be short-term side effects, eg. swelling and redness at the area of injection and feeling unwell for a week or two.

You can't have some vaccines if you're already ill, especially if your immune system is weakened.

Some people think that immunisation can cause other disorders, eg. one study suggested a link between the MMR (measles. mumps and rubella) vaccine and autism. Scientist now know it's perfectly safe.

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Antibiotics

What are antibiotics?

Antibiotics are drugs that kill bacteria without killing your own body cells. They're very useful for clearing up bacterial infections that the body can't handle, however they don't kill viruses.

What are antivirals?

Antivirals can be used to treat viral infections. Antivirals are drugs that stop viruses from reproducing.

What are resistant bacteria?

A bunch of bacteria, including a resistant variety get bathed in antibiotics.

Most of the normal bacteria die.

The resistant bacteria multiply and become more common.

Eventually, the entire infection evolves into a resistant strain.

Misuse of antibiotics (eg. doctors over prescribing them or

patients not finishing a course) has increased the rate of development

of resistance.

MRSA is the best known example of antibiotic-resistant

strain.

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Drug Development

Drugs (eg. antibodies and antivirals) are substances that cause changes to the body.

New medical drugs have to be tested

to ensure that they work, and are safe, before they can be prescribed.

There are three main stages of testing.

The drugs are tested

using computer models and human cells grown in a laboratory. Many

substances fail this test because they damage cells or do not seem to

work.

Drugs that pass the first stage are tested on animals. In

the UK, new medicines have to undergo these tests. But it is illegal to

test cosmetics and tobacco products on animals. A typical test involves

giving a known amount of a substance to the animals, then monitoring

them carefully for any side-effects.

Drugs that have passed

animal tests are used in clinical trials. They are tested on healthy

volunteers to check that they are safe. Very low doses of the drug are

given to begin with. If there are no problems, further clinical trials

are done to find the optimum dose for the drug.

Clinical

trials are not without risk. Sometimes severe and unexpected

side-effects occur. Most substances do not pass all of the tests and

trials, so drug development is expensive and takes a long time.

What is a blind trial?

It

is important to be certain that a drug really does have positive

effects, rather than people feeling better because they expect to

feel better if they take a medicine. This is called the ‘placebo

effect’.

Blind trials aim to minimise the placebo effect.

A placebo

is designed to appear exactly the same as the drug itself, but it does

not actually contain any of the drug.

Some

patients are given the drug while others are given a placebo. The patients are not

told who have received the drug and who have received the placebo until

the trial is over.

What is a double blind trial?

It is exactly the same as a blind trial except both the doctors and the patients do not know who have received the placebo or drug.

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Cancer

What is cancer?

A cancer happens when cells begin to divide out of

control. They form tumours that can sometimes be felt as an unusual lump

in the body.

What are tumours?

Tumours can be benign or malignant,

benign tumours grow slowly and are usually harmless – warts are benign tumours

malignant tumours often grow more quickly and may spread throughout the body. They can be fatal.

How does diet and lifestyle affect the risk of developing cancer?

smoking increases the risk of lung cancer

using sunscreen reduces the risk of skin cancer

eating more fruit and vegetables reduces the risk of bowel cancer.