Communicable Diseases - Biology

·         Pathogens are microorganisms that cause infectious disease.

·          Pathogens may be viruses, bacteria, protists or fungi.

·         They may infect plants or animals

·          They can be spread by direct contact, by water or by air.

·         Bacteria and viruses may reproduce rapidly inside the body.

·         Bacteria may produce poisons (toxins) that damage tissues and make us feel unwell

·         Viruses live and reproduce inside cells, causing cell damage.

·         Measles is a viral disease showing symptoms of fever and a red skin rash. Measles is a serious illness that can be fatal if complications arise. For this reason most young children are vaccinated against measles.

·         The measles virus is spread by inhalation of droplets from sneezes and coughs.

·         HIV initially causes a flu-like illness. 

·         Unless successfully controlled with antiretroviral drugs the virus attacks the body’s immune cells.

·         Late stage HIV infection, or AIDS, occurs when the body’s immune system becomes so badly damaged it can no longer deal with other infections or cancers. HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles.

·         Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes.

·         It gives a distinctive ‘mosaic’ pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis.

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

·         In the UK, poultry are vaccinated against Salmonella to control the spread.

·         Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete.

·         Gonorrhoea is a sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge from the vagina or penis and pain on urinating.

·         It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared.

·         Gonorrhoea is spread by sexual contact. The spread can be controlled by treatment with antibiotics or the use of a barrier method of contraception such as a condom.

·         Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early.

·         It affects the growth of the plant as photosynthesis is reduced.

·         It is spread in the environment by water or wind.

·         Rose black spot can be treated by using fungicides and/or removing and destroying the affected leaves

·         The pathogens that cause malaria are protists.

·         The malarial protist has a life cycle that includes the mosquito.

·         Malaria causes recurrent episodes of fever and can be fatal.

·         The spread of malaria is controlled by preventing the vector, mosquitos, from breeding and by using mosquito nets to avoid being bitten, or eliminating their place of breeding.

·         Non-specific defence systems of the human body against pathogens include the skin, nose, trachea and bronchi and stomach

·         If a pathogen enters the body the immune system tries to destroy it. 

• The skin is an impermeable barrier. 
• The nose has hairs to protect itself from pathogens. Mucus traps the pathogens and pushes them to the back of the throat, which has cilia to trap them.
• The trachea has silia to push the mucus up and swallow it. 
• The stomach has hydrochloric acid to break down the pathogens. 

White blood cells help to defend against pathogens by:

• Phagocytosis: Engulfs and digests the pathogen by producing an enzyme.

• Antibody production: Each pathogen has different surface appearances, and therefore different antigens. An antibody that is complementary to the antigen is produced, and they clump together, so that they can be engulfed and digested.

• Antitoxin production: Produced to stick to the toxins and neutralize or inactivate their effects. Then they are removed by the phagocytes.

·         Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies.

·         If the same pathogen re-enters the body the white blood cells respond quickly to produce the correct antibodies, preventing infection.

·         spread of pathogens can be reduced by immunising a large proportion of the population, which is called herd immunity.

·         Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body.

·         It is important that specific bacteria should be treated by specific antibiotics.

·         The use of antibiotics has greatly reduced deaths from infectious bacterial diseases.

·         However, the emergence of strains resistant to antibiotics is of great concern.

·         Antibiotics cannot kill viral pathogens.

·         Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens.

·         It is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

·         Traditionally drugs were extracted from plants and microorganisms.

·         The heart drug digitalis originates from foxgloves.

·         The painkiller aspirin originates from willow trees.

·         The antibiotic penicillin is produced by a fungus called Penicillium mould.

·         Penicillin was discovered by Alexander Fleming.

·         Most new drugs are synthesised by chemists in the pharmaceutical industry. However, the starting point may still be a chemical extracted from a plant.

·         New medical drugs have to be tested and trialled before being used to check that they are safe and effective. Drugs are extensively tested for toxicity, efficacy and dose.

·         The stages of testing a drug are:

·         Preclinical testing is done in a laboratory using cells, tissues and live animals.

·         Clinical trials use healthy volunteers and patients.

·         Clinical trials involve:

• Very low doses of the drug are given at the start of the clinical trial.
• If the drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug.

·         Healthy volunteers are given the drug in a double blind trial; some patients are given a placebo.