Communicable Diseases & Human Defense Systems

Communicable (Infectious) Diseases

Definition and Examples

• A communicable disease, also known as an infectious disease, is caused by a pathogen and can be transmitted from infected to uninfected individuals.
• Examples include cholera, HIV/AIDS, malaria, and tuberculosis.
• Non-infectious diseases are long-term illnesses not caused by pathogens, such as lung cancer, cardiovascular disease, depression, cystic fibrosis, and vitamin deficiencies.

Pathogens and Transmission

• Many microorganisms can be pathogens, including:
  • Bacteria: reproduce rapidly and produce toxins.
  • Viruses: reproduce rapidly within host cells, causing cell damage.
  • Fungi.
  • Protoctists.
• Transmission occurs through direct contact, contaminated water, or airborne droplets.
• Prevention involves stopping the spread of pathogens through good hygiene and effective sanitation.

Measures to Prevent Spread

Viral Diseases

Basics

• Viruses are common causes of diseases like the common cold and flu.
• Viruses are not considered living organisms because they do not fulfill the 7 life processes.
• Viruses lack nuclei, organelles, and cytoplasm.
• They reproduce rapidly by inserting genetic material into host cells to create new viral particles.
• The host cell bursts open, releasing viral particles to infect other cells.

Measles

• A highly contagious and potentially serious viral infectious disease.
• Most often seen in children, who are typically vaccinated against it.

HIV

• HIV (Human Immunodeficiency Virus) can lead to Acquired Immunodeficiency Syndrome (AIDS).

Tobacco Mosaic Virus

• The first virus to be isolated by scientists.
• A widespread plant pathogen that infects about 150 species of plants, including tomato plants and cucumbers.

Bacterial Diseases

Basics

• Bacterial pathogens are cells that can infect plants and animals.
• Not all bacteria are harmful; some are beneficial.
• Bacteria on the skin compete with harmful pathogens.
• Bacteria in the large intestine digest substances like cellulose and provide essential nutrients like Vitamin K.
• Bacterial pathogens produce toxins that damage cells and tissues directly.
• Under optimal conditions (warmth, moisture, nutrients), some bacteria reproduce rapidly; some species of E. coli can reproduce once every 20 minutes.

Salmonella

• Salmonella food poisoning is spread by bacteria ingested in food or on food prepared in unhygienic conditions.
• Found in the gut of many different animals.

Gonorrhoea

• A sexually transmitted disease (STD), also known as a sexually transmitted infection (STI).
• In 2018, it had the largest increase (26%) of infections in the UK, with a total of 56,259 cases, accounting for 13% of diagnosed STDs.

Fungal Diseases

Basics

• Few fungal diseases affect humans; an example is athlete's foot, spread by contact with contaminated surfaces.
• Fungi can be unicellular (e.g., yeast) or have a body made of thread-like structures called hyphae.
• Hyphae can grow and penetrate plant and animal surfaces, causing infections.
• Hyphae produce spores that spread infection to other organisms.
• Fungal infections are more common in plants and can destroy crops.

Rose Black Spot

• A fungal disease of plants where purple or black spots develop on leaves, which often turn yellow and drop early.
• Affects plant growth by reducing photosynthesis.
• Spread by water or wind.
• Treated by using fungicides and/or removing and destroying affected leaves.

Protist Diseases

Basics

• Protists are a diverse group of eukaryotic and usually unicellular organisms.
• Only a small number of protists are pathogenic, but the diseases they cause are often serious.
• Often need a vector to transfer from one host to the next.

Malaria

• The pathogens that cause malaria are protists from the Plasmodium family (four species).
• The malarial protist has a life cycle that includes the mosquito as a vector.
• Malaria causes recurrent episodes of fever and can be fatal.
• The spread of malaria is controlled by preventing mosquito breeding and using mosquito nets.
• Part of the malaria life cycle is in humans, and the other part is in mosquitos.

Human Defence Systems

Non-Specific Defences

• The human body has mechanisms as the first line of defence against infection:
  • The skin
  • The nose
  • The trachea and bronchi
  • The stomach
• These mechanisms can be divided into biochemical and physical defenses.

The Immune System

• The immune system of the body is highly complex, with white blood cells being the main component.
• Once a pathogen has entered the body, the role of the immune system is to prevent the infectious organism from reproducing and to destroy it.
• White blood cells help defend against pathogens by:
  • Phagocytosis
  • Production of antibodies
  • Production of antitoxins

Phagocytosis

• Phagocytes engulf and digest pathogens; this can be non-specific or helped by antibodies, which cause agglutination (clumping) of pathogens.
• The phagocyte surrounds the pathogen and releases enzymes to digest and break it down to destroy it.

Production of Antibodies

• Lymphocytes produce antibodies.
• Antibodies are Y-shaped proteins; each individual can make millions of different types, each with a slightly different shape.
• The aim of antibody production is to produce the antibody that is specific to the antigens on the surface of the pathogen.
• This is a specific type of immune response, as the antibodies produced are specific to each pathogen's antigens.
• It can take a few days to make the antibodies that are specific to a pathogen, giving the pathogen time to make you feel unwell.
• Memory cells are lymphocytes that remain in the body after an initial infection; they produce specific antibodies so that if you get infected by the same pathogen again, you can produce antibodies much quicker.

Production of Antitoxins

• Some pathogens (usually bacteria) produce substances that act as toxins which make you feel unwell.
• Lymphocytes can produce antibodies against these substances – in this case, they are called antitoxins.
• The antitoxins neutralise the effects of the toxin.

Key Definitions

• Antigen: A molecule found on the surface of a cell.
• Antibody: A protein made by lymphocytes that is complementary to an antigen and, when attached, clumps them together and signals the cells they are on for destruction.
• Antitoxin: A protein that neutralises the toxins produced by bacteria.

Vaccination

Why Vaccinate?

• Vaccination prevents illness by providing artificial immunity.
• It involves exposing an individual to the antigens of a pathogen in some form, triggering an immune response that results in the formation of memory cells.
• If a vaccinated individual is infected with the pathogen, they can destroy it before they become infectious.
• Vaccines reduce the likelihood that an infected individual will spread the pathogen.
• If a large number of the population are vaccinated, it is unlikely that an unvaccinated individual will become infected; this is the principle behind herd immunity.

Herd Immunity

• Protects the vulnerable that may not be able to have the vaccine.

Worldwide Vaccination

• The role of the WHO is to monitor global diseases and track if a disease is endemic, epidemic, or pandemic.
• The number of people with measles worldwide is increasing due to a drop in the vaccination rate.

How Vaccines Work

• 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.

Types of Immunity

• Active immunity: comes from the body creating antibodies to a disease either by exposure to the disease (natural) or by vaccination (artificial).
• Passive immunity: comes from antibodies given to you from another organism, for example, in breast milk.
• Vaccination with a weakened form of the measles virus results in the production of antibodies and memory cells.
• When exposed to the virus naturally and infected, an individual can produce a higher concentration of antibodies much more quickly to destroy it.

Antibiotics & Painkillers

Treatment Types

• Medicines that treat the cause of the disease (e.g., antibiotics).
• Medicines that treat the symptoms of the disease (e.g., painkillers).

Antibiotics

• Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body.
• The use of antibiotics has greatly reduced deaths from infections.
• Only certain antibiotics work on certain diseases, so a doctor will prescribe different antibiotics depending on the infection type.
• Antibiotics work by stopping bacteria cellular processes, such as the production of the cell wall.
• Penicillin was the first antibiotic to be discovered and is widely used, although resistance is a problem.
• Antibiotics will not work against viruses, as viruses reproduce inside cells.

Painkillers

• Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens (e.g., ibuprofen can reduce pain and inflammation).

Antibiotic Resistance

• The use of antibiotics has increased exponentially since they were first introduced in the 1930s.
• The introduction of antibiotics has had one of the largest impacts on global health.
• Antibiotic resistance has developed in many different types of bacterial species due to overuse.
• Bacteria have random mutations in their DNA; some mutations may give them resistance to an antibiotic.
• If an organism is infected with bacteria and some of them have resistance, they are likely to survive treatment with antibiotics.
• The population of the resistant bacteria will increase.
• If the resistant strain is causing a serious infection, then another antibiotic will be needed.
• A strain of Staphylococcus aureus has developed resistance to a powerful antibiotic methicillin; this is known as MRSA (Methicillin-resistant Staphylococcus aureus).
• MRSA can infect wounds and is difficult to treat without antibiotics.

Preventing Resistant Bacteria

• Doctors need to avoid the overuse of antibiotics, prescribing them only when needed.
• Antibiotics shouldn't be used in non-serious infections that the immune system will ‘clear up’.
• Antibiotics shouldn't be used for viral infections.
• Patients need to finish the whole course of antibiotics so that all the bacteria are killed and none are left to mutate to resistant strains.
• Antibiotics use should be reduced in industries such as agriculture.

Reducing the Spread of Resistant Strains

• Good hygiene practices, such as handwashing and the use of hand sanitisers, have reduced the rates of resistant strains of bacteria, such as MRSA, in hospitals.
• The isolation of infected patients to prevent the spread of resistant strains, in particular in surgical wards where MRSA can infect surgical wounds.

Discovery & Development of Drugs

Discovering New Drugs

• Traditionally, drugs were extracted from plants and microorganisms.
• New drugs are being developed all the time by scientists at universities and drug companies around the world.
• Lots of the medications that we use today are based on chemicals extracted from plants (e.g. digitalis from foxgloves, aspirin from willow).
• Penicillin was discovered by Alexander Fleming from the Penicillium mould.
• Most new drugs are synthesised by chemists in the pharmaceutical industry; however, the starting point may still be a chemical extracted from a plant.

Testing New Drugs

• All new drugs need to be tested and trialled before they can be used in patients.
• They are tested for:
  • Toxicity – does it have harmful side effects?
  • Efficacy – does the drug work?
  • Dose – what dose is the lowest that can be used and still have an effect?
• The results of any testing are then peer-reviewed to make sure that the results are described accurately.
• The results would then be published in journals.

Developing New Drugs

• Preclinical testing is done in a laboratory using cells, tissues, and live animals.
• Clinical trials use healthy volunteers and patients.
• 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.
• In double-blind trials, some patients are given a placebo.

3 Stages of Drug Development

Future Medications

• Pharmaceutical companies are always looking to find new medications, including:
  • Vaccinations to different diseases
  • Antibiotics that have a different action on the bacteria so that bacteria are not resistant to them
  • Painkillers with fewer side effects
  • Antiviral drugs that don’t damage the body’s tissues
• Sources of these medications may be plants or microorganisms.