12. Disease


The general course of a disease: 


  • Droplet infection – coughs, sneezes e.g. cold, influenza • Drinking contaminated water – e.g. cholera, typhoid
  • Eating contaminated food – e.g. polio, salmonella
  • Direct contact – skin to skin contact e.g. athlete’s foot, ringworm
  • Sexual intercourse – e.g. AIDS, syphilis, cklamydia
  • Blood to blood contact – e.g. AIDS, hepatitis B
  • Animal vectors – e.g. malaria, sleeping sickness

2. Incubation period:

The time between when a person is infected and when they first show symptoms. This occurs as the pathogen may need time to multiply for the effects to become large enough. It also may need time to reach its destination.


A disease that can be seen by other people. It can be seen, heard or be measured (e.g. blood sugar). A sign is different from a symptom as a symptom does not have to be visible to other people as it is what the patient experiences.

Endemic disease – a disease that is always present in the population of a particular geographical area.

Epidemic disease – a widespread outbreak of a disease spreading over a large area and many people.

Pandemic disease – a worldwide outbreak of a disease e.g. Swine Flu (2009).





A virus takes over the host cell and its genetic machinery and uses it to make more virus particles. The host dies after more viruses are reproduced and the viruses then spread to other cells in the body.


  • Influenza: transmitted through airborne droplets produced when a person sneezes or coughs. Influenza primarily affects the cells in the upper airways of the respiratory system. There is little that can be done to treat the disease although antibiotics can be used to combat the secondary bacterial infections. The disease can however be prevented 60-70% of the time through vaccinations. Staying away from infected people also prevents transmission.
  • Poliomyelitis: this is spread through animal vectors such as flies and also through contaminated water and food. There is no effective treatment but this can be prevented through a vaccine and/ or a good hygiene so that flies do not make contact with human sewage or food and drink.
  • AIDS: this is spread through the HIV virus through sexual intercourse or blood to blood contact. There is no cure or vaccine. It can be prevented through not sharing needles or limiting the number of sexual partners.




Most bacteria are heterotrophic meaning they live off other organisms and eat organic matter. Saprobes are bacteria that live off dead organic material while heterotrophic parasites are those that cause disease. Some bacteria are also autotrophic meaning they produce their own food through photosynthesis.


Bacteria reproduce asexually through binary fission. This is where the DNA of a bacterium duplicates creating two daughter bacteria with the same genetic information as the parent bacterium.


  • Typhoid: spread through contaminated water containing the bacteria Salmonella Typhi or flies transferring the bacteria from faeces to food. It can be treated with antibiotics such as penicillin and the oral rehydration method is useful in combating the effects. It can be prevented through a vaccine, better sanitisation and better hygiene.
  • Tuberculosis (TB): this is spread through droplet infection of a bacteria called mycobacterium tuberculosis. Long term use of antibiotics can treat the illness but it can take up to 15 months to treat it. In that time the bacteria may become resistant to the antibiotics meaning it will have to constantly be changed. It can be prevented though better standards of living as if people are less crowded then there is less chance of infection. Also the vaccine BCG can be used although it only works for children.
  • Gonorrhoea: spread through sexual intercourse. It can be treated through antibiotics although resistance to them are growing. It is prevented through the use of condoms or through avoiding sexual intercourse with someone infected with the disease.



  • Thrush: transmitted through direct contact of the fungus candida albicans. It is treated with antifungal drugs and prevented through a good hygiene.
  • Athletes Foot: transmitted through direct contact it can once again be treated with anti-fungal drugs and prevented through a good hygiene.


Schistosomiasis (or Bilharzia): this is spread by the parasite Schistosome.

1. Larvae of the worms are released by fresh water snail.

2. The larvae swim in the water and penetrate the skin of people in the water.

3. The larvae develop in the body to adult worms, living inside the liver, intestine and bladder. They feed of red blood cells.

4. They mate and release eggs which pass out in faeces or urine to infect more snails.

Effects: Schistosomiasis is a long term illness (chronic). Symptoms are generally quite mild although in serious cases symptoms may include fever, chills, diarrhoea, severe rashes and blood in the urine. Organs may also become damaged and the liver, spleen and/or lymph nodes may become enlarged.


1. Treatment with drugs to kill the worms in the body.

2. Killing the fresh water snails through chemicals or introducing natural predators such as crayfish. This interrupts the life cycle.

3. Improving sanitisation also disrupts the cycle as it prevents faeces containing the worms from faeces and urine from entering the rivers and lakes and infecting the snails.

4. Health education needed to inform the villages about the dangers of going into the river.

Malaria and Typhoid: spread through animal vectors. Malaria parasites are spread through mosquitos while typhoid bacillus is spread through houseflies.


1. Mosquitos feed from an infected person’s sex cells.

2. Fertilisation occurs in female mosquitos. The zygote develops into malarial parasites.

3. Infected mosquitos infect the person.

4. Parasites enter liver cells and change form. They rupture the liver cells, enters the blood stream and infects the red blood cells.

5. The red blood cells burst, releasing more parasites and sex cells.

6. The process repeats.


1. Use of insecticides to kill mosquitos and houseflies.

2. Draining swamps and rubbish dumps where mosquitos and houseflies gather.

3. Use of drugs to target the life cycle of the malarial parasites.

4. Stocking ponds with a fish called Tilapia which feed on mosquito larvae.

5. Using insect repellants, wearing long sleeved shirts and sleeping under insect nets prevent bites from mosquitos.

6. Improving hygiene and sanitisation.


Immunity: immunity can be…

  • Natural – created through organic processes.
  • Artificial – created through man made intervention.
  • Active – created through an immune response and so is in the long term.
  • Passive – created without an immune response. Normally this only happens twice in our lives. First when we receive antibodies across the placenta and second through our mothers in colostrum and breast milk.


These are a form of an artificial active immunity and works by injecting a person with an “agent” that carries the same antigens as a specific disease causing microorganism. This can be achieved by injecting…

  • An attenuated (weakened) strain of the actual microorganism (e.g. polio, TB and measles)
  • Dead microorganisms (e.g. whooping cough, typhoid)
  • A modified toxin of the bacteria (e.g. tetanus)
  • Just the antigen (e.g.influenza)
  • Harmless bacteria, genetically engineered to carry the antigen of a different disease carrying microorganism.

The Antibody/Antigen reaction:

1. Lymphocytes recognise individual marker chemicals called antigens on the surface of the pathogens.

2. Lymphocytes’ receptor proteins bind with the antigens.

3. When it binds the lymphocytes divide rapidly, producing millions of the same type of lymphocyte that is capable of recognising the microorganism.

4. Most of this occurs with B and T lymphocytes.

5. Most B-lymphocytes begin to produce antibodies which bind with the antigens, causing the pathogens to clump together. This makes it easier for phagocytes to ingest it through phagocytosis where pseudopodia encloses the pathogen. Some antibodies also cause the pathogens to burst apart. Some also develop into memory cells which remain  for a long time and if the cells re-infect, the memory cells will start to reproduce and produce antibodies. Because of this the secondary immune response is much faster than the primary.

6. T-lymphocytes destroy our own cells. These cells have become infected with a virus or are cancerous. This is done through releasing chemicals that “punch a hole” in the cell or activates a “programmed cell death” that is put into the genetic coding of every cell. Some also become memory cells like B-lymphocytes.


  • Source – antibiotics such as penicillin are created by fungi. For example, penicillin is excreted by the fungus penicillium.
  • Role – bacteria can be stopped by antibiotics. Antibiotics can be both bactericidal where bacteria are killed or bacteriostatic where the bacteria is stopped from multiplying. For example, penicillin and tetracycline are bactericidal while nalidixic acid is bacteriostatic.
  • How – penicillin works by weakening cell walls by interfering with the manufacture of bacteria cell wall. Water therefore enters through osmosis and bursts the cell. Nalidixic acid interferes with DNA replication meaning bacteria cannot multiply. Tetracycline interfered with protein synthesis meaning no enzymes can be made to control the cell.

Non-pathogenic organisms and their importance:

Non-pathogenic bacteria and fungi are useful to humans because they are decomposers which break down complex organic materials into simple substances. These are then released into the environment. For example, decomposers are used to break down protein into ammonia and then nitrates which are essential for plants.

Decomposers are involved in sewage treatment. Sewage must be treated as they contain pathogenic bacteria which can cause diseases if drunk and also because aerobic bacteria in the water will deplete the amount of oxygen in the water by breaking down the organic material in the sewage. This then causes death to species not adapted to low oxygen levels. The sewage must therefore be treated first to remove any organic material.

There are two ways of treating sewage. The percolating (biological) filter method is one of them and works like this:

1. Sewage is screened to remove any large objects and left to stand in a large setting tank to allow other solid material to settle out.

2. The sewage is then pumped through sprinklers rotating over a filter bed. The filter bed contains bacteria, fungi, and protozoa which oxidises any organic material.

3. The treated sewage is discharged into a waterway.


The second method is called the activated sludge method:

1. Sewage is screened and stood in a large settling tank.

2. It is then passed into an aeration tank which when oxygen is pumped in, allows the bacteria to oxidise the organic material.

3. It passes to a sedimentation tank where the activated sludge settles.

4. Some are returned to the aeration tank carrying bacteria. The purified effluent is discharged.

A pit latrine is used in less developed countries and is basically a hole in the ground with a pit underneath containing microorganisms which can break down the urine and faeces.

All these different methods of sewage treatment rely on both aerobic and anaerobic microorganisms. Aerobic microorganisms are used to oxidise any organic matter in the sewage creating an effluent that contains much less organic material and fewer pathogens.

Anaerobic microorganisms are needed to treat the waste sludge that settles in the settling tanks. The microorganisms are placed with the sludge in a fermentation tank and the organic material are converted to biogas, a mixture of methane and carbon dioxide. The biogas can be used as a fuel in electricity generator or for heating. The remaining dry, solid material can be used for fertiliser or disposal of in a landfill site.

Eutrophication occurs when excess minerals such as nitrates and phosphates enter a body of water from sewage or fertilisers. Fertilisers can enter the water through leeching as nitrates and such are washed out of the soil by rain since it dissolves in water. This can also occur through surface run offs. Excess minerals stimulates the growth of algae. An algal bloom will develop and block out the light needed for photosynthesis and are also decomposed as they die. This is done by aerobic bacteria which uses up oxygen in the water. This causes oxygen depletion, causing many fish and plants to die. In severe cases, the water will become anoxic (containing very little oxygen) and become smelly from the gases such as hydrogen sulphide and methane which are released by the bacteria. Only anaerobic bacteria can survive conditions like these.