Communicable Diseases

• Pathogen

  Microorganisms that cause infectious disease.

  Can either be bacteria, viruses, protists or fungi.
• Ways pathogens spread

  Direct contact; touching contaminated surfaces.

  Sexual contact.

  Placental transfer; mother to foetus through placenta.

  Vector organisms; transfer pathogen from host to host.

  Droplet infection; mucus droplets containing pathogen are coughed or sneezed out then inhaled.

  Water and food; pathogen infects water or uncooked food then ingested by organism.
• Ways to prevent spread of pathogens

  Destroying vectors; pesticides or habitat destruction.

  Simple hygiene; washing hands regularly, using handkerchiefs when sneezing and coughing.

  Isolation of infected individuals.

  Vaccination.
• Virus

  Reproduce rapidly by inserting their genetic material into host cells and creating protein capsules to build new viral particles.

  Once many copies have been made, the host cell bursts and releases viral particles which go on to infect other cells.
• Measles

  Viral infectious disease.

  Can be fatal if complications rise.

  SYMPTOMS: Fever, red skin rash.

  HOW IT'S SPREAD: Inhalation of droplets from coughs or sneezes.

  TREATMENT: Vaccinating children from a young age.
• HIV

  SYMPTOMS: Starts with a flu-like illness. If untreated will start to attack the body's immune cells. Late stage HIV infection (AIDS) occurs when body's immune system is so badly damaged it can no longer deal with other infections.

  HOW IT'S SPREAD: Sexual contact, exchange of bodily fluids e.g. blood when drug users share needles.

  TREATMENT: Use of antiretroviral drugs in early stages to slow or halt progress to AIDS.
• Tobacco mosaic virus (TMV)

  Plant virus that affects many species of plants including tomatoes.

  SYMPTOMS: Distinctive mosaic pattern of discolouration on leaves. The plant will not grow as much due to the lack of photosynthesis.

  HOW IT'S SPREAD: Direct contact with infected plants, transmitted from the soil by the roots.

  TREATMENT: No treatment. Spread is controlled with good field hygiene.
• Bacteria

  Produce toxins that damage cells and tissues making us feel ill.

  Reproduce rapidly by binary fission.
• Salmonella

  Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions.

  SYMPTOMS: Fever, abdominal cramps, vomiting and diarrhoea.

  HOW IT'S SPREAD: Contaminated food that has not been cooked properly e.g. raw meat and eggs.

  TREATMENT: Poultry in the UK are vaccinated. Proper hygiene in food preparation. Cooking raw meat and eggs thoroughly.
• Gonorrhoea

  Sexually transmitted disease (STD).

  SYMPTOMS: Thick yellow or green discharge from penis or vagina, pain when urinating.

  HOW IT'S SPREAD: Unprotected sexual contact.

  TREATMENT: Barrier methods of contraception e.g. condom, antibiotics however some strains have become resistant.
• Fungi

  Often unicellular, but are made up of thread-like structures called hyphae.

  Hyphae grow and penetrate surface of plants and animals causing infection.

  Hyphae produce spores which spread infection to other organisms.
• Rose black spot

  Fungal disease in plants.

  SYMPTOMS: Purple or black spots on leaves. Leaves turn yellow then drop early. Less leaves means less photosynthesis so growth is affected.

  HOW IT'S SPREAD: Spores from fungus are carried by wind or water.

  TREATMENT: Fungicide chemicals, infected leaves removed and destroyed to prevent spread.
• Protists

  Eukaryotic and usually unicellular organisms.

  Often need a vector to transfer from one host to the next.
• Malaria

  Malarial protist has a life cycle including the mosquito as a vector.

  SYMPTOMS: Recurrent episodes of fever that can be fatal.

  HOW IT'S SPREAD: By the mosquito as a vector.

  TREATMENT: Controlling mosquito populations by using insecticides, mosquito nets to prevent biting, preventing mosquitos from breeding.
• Non-specific defence systems in humans against pathogens

  The skin; acts as a physical barrier, produce antimicrobial secretions to kill pathogens.

  The nose; has hairs and mucus to prevent pathogens entering lungs.

  Trachea and bronchi; secrete mucus to trap pathogens, cilia waft mucus upwards to be swallowed.

  Stomach; produces hydrochloric acid that kills pathogens in mucus, food and drink.
• Immune System

  Acts to destroy any pathogens that enter the body.

  White blood cells are the main component. They defend the body by:

  1: Phagocytosis

  2: Production of antibodies

  3: Production of antitoxins
• Phagocytosis

  Phagocytes engulf and digest pathogens.

  Phagocyte surrounds the pathogen and releases enzymes which digest and break it down to destroy it.
• Production of antibodies

  Lymphocytes produce antibodies.

  There are millions of different types of antibodies.

  Each lymphocyte only produces one type of antibody.

  Each antibody is specific/complementary to the antigen found on the surface of a certain pathogen.

  The antibody binds to the antigen of the pathogen and causes agglutination (clumping) of pathogens to enhance phagocytosis.
• Memory cells

  It can take a few days to produce the antibodies that are specific to a pathogen and this may give the pathogen time to make you feel unwell.

  Memory cells are types of lymphocytes that remain in the body after infection with a particular pathogen.

  They produce the specific antibodies against the pathogen so that if you get infected by the same pathogen again in the future, you can produce antibodies much quicker before it can cause harm to tissues.
• Production of antitoxins

  Lymphocytes can produce antibodies against the toxins produced by bacteria: these are called antitoxins.

  Antitoxins neutralise the effects of the toxins produced by bacteria.
• Vaccination

  Involves introducing small quantities of dead or inactive forms of a pathogen to stimulate an immune response and the formation of memory cells.

  If the same pathogen re-enters the body, the memory cells can respond quickly to produce correct antibodies, preventing infection.

  The individual is now immune.
• Herd immunity

  Vaccination reduces the likelihood that infected individuals will spread the pathogen to others.

  If a large number of the population are vaccinated, it is unlikely that an unvaccinated individual will become infected.
• Advantages and disadvantages to vaccination

  Advantages: Diseases that were once common are now rare due to widespread vaccinations and herd immunity. This can prevent epidemics from occurring.

  Disadvantages: Vaccines do not always provide immunity and can have side effects.
• Antibiotics and painkillers

  Antibiotics, like penicillin, are medicines that treat the cause of bacterial disease by killing infective bacteria in the body.

  Only specific antibiotics work on specific diseases, so it is important that specific bacteria are treated with specific antibiotics.

  Antibiotics cannot kill viral pathogens because they reproduce inside cells.

  It is difficult to develop drugs that can kill viruses without damaging tissues.

  Painkillers and other medicines treat the symptoms of disease but do not kill pathogens.
• Antibiotic resistance

  Bacteria have random mutations in their DNA.

  One of these mutations may give them resistance to an antibiotic.

  If an organism is infected with bacteria and some of them possess this mutation, they are likely to survive treatment with the antibiotic.

  The population of the resistant bacteria will increase and the individual will have to take another antibiotic to kill them.

  Bacteria that have become resistant to multiple types of antibiotic are called superbugs, an example is MRSA.
• Preventing resistant bacteria

  Doctors need to avoid overuse of antibiotics by prescribing them only when needed.

  Patients must finish the full course of antibiotics so that all the bacteria are killed and none are left to mutate to resistant strains.

  Antibiotic use should be reduced in agriculture.
• Reducing spread of resistant strains

  Good hygiene practices.

  Isolation of infected patients, particularly in surgical wards where MRSA can infect surgical wounds.
• Discovery of drugs

  Drugs were traditionally extracted from plants and microorganisms. Now, drugs are being developed on large scales in the pharmaceutical industry, but the starting point may still be a chemical extracted from a plant.

  The heart drug digitalis originates from foxgloves.

  The painkiller aspirin originates from willow.

  Penicillin was discovered by Alexander Fleming from the Penicillium mould.
• Testing of drugs


  Drugs need to be tested for:

  1: Toxicity - Does it have harmful side effects?

  2: Efficacy - Does the drug work?

  3: Dose - What dose is the lowest that can be used while still having an effect?

  Results of the tests are then peer-reviewed to ensure accuracy then published.
• Stages in drug testing

  1. Preclinical testing

  2. Clinical trials
• Preclinical testing


  • Drug is tested on cells and tissues in the lab
  • Drug is tested on animals to see its effect on a whole organism
• Clinical trials

  1. Drug is first tested on healthy human volunteers with a low dose at first, which is then increased, to ensure safety

  2. Drug is then tested on patients with the condition in a double-blind study

  3. Patients are split into two groups, one is given the drug and the other is given a placebo

  4. Neither the doctor or patient knows if they are getting the drug or placebo

  5. Once the drug is found to be safe then the lowest dose effective is tested