Revision notes · Infection and response
Communicable diseases
Communicable (infectious) diseases4.3.1.1
Definition: Communicable diseases are infectious illnesses caused by pathogens that can be passed from one organism to another.
Pathogens cause infectious diseases in both plants and animals.
- •Pathogens are microorganisms that cause infectious disease, including viruses, bacteria, protists, and fungi.
- •Bacteria reproduce rapidly and produce toxins that damage tissues and make us feel ill.
- •Viruses live and reproduce inside host cells, causing cell damage when they burst out.
- •Pathogens can spread by direct contact, water, or air (droplets).
- •Disease spread can be reduced by hygiene, isolation, controlling vectors, and vaccination.
⚠️ Common mistake: Claiming that bacteria reproduce inside host cells and cause damage by bursting them, which is actually how viruses cause damage.
🧠 Remember: Bacteria = Toxins; Viruses = Inside Vital cells.
Viral diseases4.3.1.2
Definition: Viral diseases are infections caused by viruses that invade host cells and replicate inside them.
Viruses damage host cells when they replicate inside them and cause severe diseases.
| Disease | Symptoms | Spread & prevention |
|---|---|---|
| Measles | Fever, red skin rash | Inhaled droplets from coughs/sneezes; most children are vaccinated with the MMR vaccine |
| HIV | Initial flu-like illness; late stage (AIDS) leaves the immune system unable to fight other infections | Sexual contact or exchange of bodily fluids; controlled with antiretroviral drugs |
| Tobacco mosaic virus (TMV) — plants | 'Mosaic' discolouration on leaves, reducing photosynthesis and growth | Spreads between plants on contact; insects can also act as vectors |
⚠️ Common mistake: Believing that antibiotics can be used to treat viral infections like measles or HIV.
🧠 Remember: TMV = Totally Messy Vision on leaves (reduces photosynthesis).
Bacterial diseases4.3.1.3
Definition: Bacterial diseases are infections caused by pathogenic bacteria that release toxins into the host.
Bacteria multiply rapidly inside the body and produce toxins that make us feel ill.
| Disease | Symptoms | Spread & prevention |
|---|---|---|
| Salmonella (food poisoning) | Fever, abdominal cramps, vomiting, diarrhoea — caused by bacterial toxins | Unhygienic food prep or undercooked poultry; UK poultry are vaccinated to control spread |
| Gonorrhoea (STI) | Thick yellow or green discharge, pain when urinating | Sexual contact; treated with antibiotics, prevented with barrier contraception (condoms) |
⚠️ Common mistake: Thinking Salmonella symptoms are caused by bacteria directly destroying cells, rather than the harmful toxins they release.
🧠 Remember: Salmonella = Sick tummy from toxins; Gonorrhoea = Green discharge.
Fungal diseases4.3.1.4
Definition: Fungal diseases are plant or animal infections caused by fungal pathogens made of eukaryotic cells or hyphae.
Fungal pathogens infect plants and animals, often spreading via spores in wind or water.
- •Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop off.
- •Rose black spot reduces the amount of photosynthesis that can occur, stunting plant growth.
- •The fungus is spread in the environment by water splashing or by the wind.
- •Rose black spot can be treated using fungicides and/or removing and destroying infected leaves.
⚠️ Common mistake: Assuming rose black spot kills the plant directly, rather than stunting growth by reducing photosynthesis.
🧠 Remember: Black spot = Fungicide or Pluck the leaf.
Protist diseases4.3.1.5
Definition: Protist diseases are infections caused by single-celled eukaryotic microorganisms called protists.
Protist pathogens often depend on vectors to transfer them between hosts.
- •Malaria is a disease caused by protist pathogens that infect blood and liver cells.
- •The malarial protist has a complex 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 vectors from breeding and using mosquito nets to avoid bites.
⚠️ Common mistake: Calling the mosquito the pathogen; the mosquito is the vector, while the protist is the actual pathogen.
🧠 Remember: Mosquito = Vector (carries it); Protist = Pathogen (causes it).
Human defence systems4.3.1.6
Definition: Human defence systems are non-specific barriers and specific white blood cell responses that protect the body against pathogens.
The body uses physical and chemical barriers as well as white blood cells to prevent and fight infection.
| Non-specific barrier | How it defends |
|---|---|
| Skin | Acts as a physical barrier to pathogens |
| Nose | Hairs and mucus trap pathogens and particles |
| Trachea & bronchi | Mucus traps pathogens; cilia sweep it away from the lungs |
| Stomach | Hydrochloric acid kills pathogens |
- •Phagocytes engulf and digest pathogens in a process called phagocytosis.
- •Lymphocytes produce specific antibodies that bind to matching antigens on pathogens to destroy them.
- •Lymphocytes produce antitoxins to neutralise toxins released by bacteria.
- •Antibodies are specific to a single type of pathogen antigen.
⚠️ Common mistake: Thinking antitoxins destroy pathogens directly, when they actually neutralise bacterial toxins.
🧠 Remember: Phagocytes = Pack-man (engulf); Lymphocytes = Lock onto antigens with antibodies.
Vaccination4.3.1.7
Definition: Vaccination is the introduction of small quantities of dead or inactive pathogens into the body to stimulate antibody production.
Vaccines immunise people against diseases by creating memory cells.
- •Vaccination introduces a dead or inactive form of a pathogen to stimulate white blood cells to produce specific antibodies.
- •If the same pathogen re-enters the body, memory cells produce antibodies rapidly to destroy the pathogen before you feel ill.
- •If a large proportion of the population is immune, the spread of the pathogen is reduced, creating herd immunity.
- •Vaccines can protect against both bacterial and viral diseases.
⚠️ Common mistake: Thinking vaccines contain active pathogens that make you sick, or that they contain ready-made antibodies.
🧠 Remember: Vaccines train Memory cells so antibodies act Fast.
Antibiotics and painkillers4.3.1.8
Definition: Antibiotics are medicines that kill infective bacteria inside the body, whereas painkillers only treat symptoms.
Antibiotics cure bacterial infections, but painkillers only treat pain without killing pathogens.
| Antibiotics | Painkillers | |
|---|---|---|
| Example | Penicillin | Paracetamol |
| What it does | Kills bacteria — cures the infection | Treats symptoms (pain, inflammation) — doesn't kill pathogens |
| Effect on viruses | None — viruses replicate inside host cells, out of reach | None — doesn't target pathogens at all |
- •Specific antibiotics must be used to treat specific bacterial infections.
- •Overuse and incomplete courses of antibiotics have created resistant strains of bacteria, such as MRSA.
⚠️ Common mistake: Stating that antibiotics can be used to treat viral infections like colds or flu.
🧠 Remember: Antibiotics = BACTERIA only; Painkillers = PAIN only.
Discovery and development of drugs4.3.1.9
Definition: Drug discovery and development is the process of finding or synthesizing chemicals and testing them for safety, efficacy, and dosage.
New drugs are discovered from natural sources or synthesised by chemists, then rigorously tested.
| Drug | Original source | Use |
|---|---|---|
| Digitalis | Foxgloves | Heart drug |
| Aspirin | Willow | Painkiller |
| Penicillin | Penicillium mould | Antibiotic |
- •Most new drugs are synthesised by chemists, though the starting point may still be extracted from a plant.
- •New medical drugs are tested for toxicity (safety), efficacy (effectiveness), and dosage (amount needed).
- •Preclinical testing is done in laboratories on cells, tissues, and live animals.
- •Clinical trials test healthy volunteers and patients at low doses for safety, then higher doses; double-blind trials use a placebo to eliminate bias.
⚠️ Common mistake: Confusing preclinical testing (cells, tissues, animals) with clinical trials (human volunteers and patients).
🧠 Remember: Testing order: Cells -> Animals -> Healthy humans -> Patients (CAHP).
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