Infection and Response

Communicable diseases are infectious. They are caused by pathogens which are passed between people in different ways: some travel through the air in coughs and sneezes, while other pathogens are passed in contaminated food or in bodily fluids.

Communicable diseases

Pathogens are microorganisms which cause disease. Diseases caused by pathogens are called communicable (infectious) diseases and they spread through a population as the pathogen passes between people. Pathogens include viruses, bacteria, fungi and protists.

Diseases that are not caused by pathogens are known as non-communicable (non-infectious diseases), such as cancer, diabetes and heart disease. They are often caused by genetics or environmental factors.

Bacteria and viruses make us feel ill in different ways. Viruses make us feel ill by entering our cells (the host cell) and replicating inside of them, causing cell damage. Bacteria make us feel by producing toxins (poisonous molecules) which can damage our tissues. Once bacteria and viruses have entered the body, they can reproduce very rapidly.

Viral diseases

Diseases caused by viruses include measles, AIDS and tobacco mosaic virus (TMV).

Measles is a disease which you were probably vaccinated for as a child. The virus spreads when a healthy person inhales infected lipid droplets from an infected person (after they sneeze or cough). Symptoms of measles include rashes and a fever.

AIDS is a disease caused by the HIV virus. The virus replicated inside cells of the immune system, destroying them as it replicates. This weakens the immune system of the infected person and prevents them from fighting off minor infections.

HIV is transmitted through the exchange of bodily fluids which could occur in the following ways:

• Unprotected sexual intercourse

• Sharing hypodermic needles

• Blood transfusions

• From mother to child during childbirth

Tobacco mosaic virus is a disease which affects plants. It causes discolouring of leaves, reducing the amount of pigment in the leaves which means the plant cannot photosynthesise as efficiently. The plant does not produce as much glucose so will display stunted growth. It is spread when the leaves of a healthy plant physically touches a plant with TMV.

Bacterial disease

Diseases which are caused by bacteria include gonorrhoea and food poisoning.

Gonorrhoea is a bacterial disease which is spread by having unprotected sex with an infected person. Symptoms include painful urination and a yellow-coloured discharge around the genitals. It is treated with antibiotics and it can be prevented by using condoms or other barrier methods.

Salmonella food poisoning is another bacterial disease — it is caused by eating food containing the Salmonella bacteria. Once the bacteria has entered our body, it divides rapidly and releases toxins which damage our cells and make us feel ill. Symptoms of food poisoning include stomach ache, vomiting and diarrhoea.

Fungal disease

Rose black spot is a fungal disease that affects plants. It causes reddish-black spots to appear on the leaves of plants. This discolouration means that the plant cannot carry out as much photosynthesis so cannot grow very well. The leaves may eventually turn yellow and fall off.

Rose black spot spreads when the fungus lands on a plant after being blown by the wind or by floating along rivers or streams. Its spread can be prevented with fungicides (chemicals which kill fungi). Gardeners will also cut off diseased leaves from a plant to reduce its spread.

Protist disease

Malaria is a disease caused by the protist Plasmodium.

Some mosquitos carry Plasmodium and transfer it to the bloodstream of a person when they bite. Symptoms include a recurring fever and sometimes death. Its spread can be prevented using mosquito nets, insect repellent and anti-malarial drugs.

Human defence systems

Humans have several barriers to stop pathogens from entering our body – these are referred to as our non-specific defences because they shield us against all types of pathogens.

Non-specific defence systems include:

• Skin – the skin forms an impenetrable barrier to pathogens. If the skin is wounded, our blood forms a clot at the site of the wound to prevent the entry of pathogens.

• Nose – the nose has a mucus lining which traps pathogens that are present in inhaled air

• Trachea and bronchi – the cells of our airways also produce mucus which traps pathogens that we breathe in. These cells also contain finger-like projections called cilia which move to waft mucus towards the stomach, where it will be destroyed by the stomach acid.

• Stomach – the stomach contains hydrochloric acid which kills any pathogens.

Specific immune response

If a pathogen enters the body, this stimulates the specific immune response. The type of response that is stimulated depends on what type of pathogen enters the body. The specific immune response defends us against pathogens in three ways:

1.    Phagocytosis – a type of white blood cell called a phagocyte engulfs the pathogen and contains it in a vesicle called a phagosome. The phagocyte also contains a lysosome which is another type of vesicle containing digestive enzymes. The phagosome and the lysosome fuse, forming a phagolysosome. The digestive enzymes digest the pathogen, killing it in the process. The debris from the digested pathogen is then removed from the phagocyte.

2.    Antibody production – lymphocytes (white blood cells) produce antibodies which bind to molecules that are found on the surface of pathogens (antigens). When antibodies bind to antigens, they stop the pathogen from being able to enter cells and causing harm.

3.    Antitoxin production – lymphocytes also produce antitoxins which bind to the toxins released by bacteria. When antitoxins are bound to toxins, they neutralise them and stop them from causing tissue damage.

Vaccination

Vaccination is a medical procedure which involves injecting someone with a dead or attenuated (weakened) form of a pathogen to stimulate white blood cells to produce antibodies.

Since the pathogen is dead or inactive, the person will mount an immune response without suffering from any symptoms. If the person is infected with the same pathogen in the future, the correct antibodies can be produced quickly and in high numbers.

We can prevent the spread of communicable diseases by vaccinating a large proportion of the population. Even if there are a minority that have not been immunised, there will be less infectious people within the population, so there is a reduced chance of the non-vaccinated people encountering an infectious person. This is known as herd immunity.

Antibiotics and painkillers

Antibiotics are medicines which kill bacteria. They are therefore used in the treatment of bacterial diseases, such as gonorrhoea. They are ineffective against diseases that are caused by viruses, such as measles and HIV/AIDS.

Unfortunately, over the past few decades, some bacteria have become resistant to certain antibiotics which means that there are less treatment options for some bacterial diseases. For example, gonorrhoea used to be treated with penicillin but now many strains are resistant so other antibiotics must be used. Prescribing antibiotics unnecessarily or not finishing the whole course of antibiotics are factors which increase antibiotic resistance. If more antibiotics are not discovered, we may reach a point where some bacteria are resistant to all the antibiotics that we have available.

Painkillers are medicines which reduce pain. They are used to treat the symptoms of the disease. They do not kill pathogens so cannot be used as a cure.

Treatments for viruses are limited because it is difficult to develop drugs which kill only viral particles and not our own cells. Vaccines are used as a preventative measure but won’t treat the disease once somebody is infected.

Drugs and development

Most of the drugs that we commonly use were derived from plants. For example, the active ingredient in aspirin originates from willow and the antibiotic penicillin was discovered from a mould growing on a melon. A drug called digitalis, which is used to treat heart failure, was extracted from foxgloves.

Modern drugs are mostly synthesised artificially by chemists in the lab, but often the compound is a variation or an artificial version of a compounds that has been extracted from plants. It’s important to maintain biodiversity as there may be lots of plant species which contain chemicals with useful medicinal properties.

Before newly synthesised drugs are made available to patients, they must first undergo preclinical and clinical testing.

• Preclinical testing involves checking the potential toxicity of the drug by adding it to cell culture in the lab or to live animals.

• The drug then goes through clinical testing, where the drug is given to both healthy volunteers and patients.

• First, a very low dosage of the drug is given to check that it is safe. If there are no adverse side effects, the dosage is increased to find the optimum dose.

• Participants will be given either the drug or a placebo. A placebo is a substance which has no medicinal value and its purpose is to rule out any possible psychological benefit an individual might feel just from receiving medical attention.

• Clinical trials are usually double-blind – this is where neither the patients nor doctors are aware who has received the drug and who has received the placebo. This is important because the doctor may subtly give away who has received the actual treatment (for example, by paying more attention to the symptoms displayed by the treatment group). Double-blind trials are more complicated to set up, but the results are more reliable.