Topic 3: Infection & Response

What is a pathogen?

A microorganism that causes infectious disease (bacteria, viruses, protists, fungi).

What is an incubation period?

The time between catching an infection and the appearance of the first symptoms.

How do viruses cause disease?

Invade host cells, replicate rapidly inside them, and cause host cells to burst and die.

How do bacteria cause disease?

Reproduce rapidly in the body and release toxins (poisons) that damage tissues.

How can pathogens spread?

1. Direct contact. 2. Air/droplet infection. 3. Contaminated water or food. 4. Vectors (animals/insects that carry pathogens).

What is Measles?

Virus; symptoms: red skin rash, fever; spread by inhalation of droplets; prevented by MMR vaccine.

What is HIV?

Virus; flu-like illness, damages immune cells leading to AIDS; spread by sexual contact/body fluids; treated with antiretrovirals.

What is Tobacco Mosaic Virus (TMV)?

Plant virus causing mosaic discoloration on leaves; reduces chlorophyll, lowering photosynthesis and growth.

What is Salmonella?

Bacteria; symptoms: fever, abdominal cramps, vomiting, diarrhoea; spread by undercooked food; prevented by poultry vaccination.

What is Gonorrhoea?

Bacteria (STD); symptoms: yellow/green discharge, pain on urinating; spread by sexual contact; treated with antibiotics.

What is Rose Black Spot?

Fungus; purple/black spots on leaves, leaves turn yellow and drop early; spread by water splash/wind; treated with fungicides.

What is Malaria?

Protist; vector: female Anopheles mosquito; symptoms: recurrent fever episodes; prevented by mosquito nets and stopping breeding.

List four non-specific human body defences?

1. Skin: Physical barrier, secretes antimicrobial oil. 2. Nose: Hairs and mucus trap pathogens. 3. Trachea/Bronchi: Mucus and cilia transport pathogens out. 4. Stomach: HCl kills swallowed pathogens.

What three roles do white blood cells play in defending against disease?

1. Phagocytosis (engulfing/digesting). 2. Antibody production (binding antigens). 3. Antitoxin production (neutralising bacterial toxins).

How do phagocytes and lymphocytes differ?

Phagocytes engulf and digest pathogens non-specifically. Lymphocytes recognise specific antigens and produce specific antibodies and antitoxins.

Why does a secondary immune response prevent illness?

Memory cells recognise the antigen rapidly, producing large quantities of specific antibodies fast enough to destroy the pathogen before symptoms develop.

How do antibody levels differ in Primary vs Secondary immune response graphs?

Primary: Slow, lower peak concentration, takes days/weeks. Secondary: Rapid, significantly higher peak concentration, sustained longer.

What is herd immunity and what amount is roughly needed?

When a large enough proportion of the population is vaccinated or immune, making it difficult for a pathogen to spread and protecting vulnerable unvaccinated individuals.

Why do vaccination rate drops lead to disease outbreaks?

Herd immunity is lost, enabling pathogens to transmit easily through non-immune individuals.

Why are drug trial participants randomly split into drug and placebo groups?

To eliminate bias and ensure both testing groups share similar baseline characteristics.

What is a placebo?

An inactive substance that looks identical to the drug being tested, used as a control to check for psychological effects.

What is a double-blind trial?

A clinical trial where neither patients nor doctors know who receives the real drug or placebo, removing doctor bias.

Why are healthy volunteers given very low doses first in clinical trials?

To test safety and monitor for adverse toxicity without risking vulnerable ill patients.

Explain how antibiotic resistance spreads rapidly in bacteria.

A random genetic mutation provides resistance. In the presence of antibiotics non-resistant bacteria die while resistant strains survive, reproduce rapidly by binary fission, and pass on the resistance allele.

• Population Shift: Because the non-resistant bacteria are dead and the resistant ones keep multiplying, the whole population eventually becomes resistant to that antibiotic.

How can antibiotic resistance be prevented?

1. Doctors should not prescribe antibiotics for mild/viral infections. 2. Patients must complete their full course. 3. Restrict agricultural use in farm animals.

What are phagocytes and how do they defend against pathogens?

A type of white blood cell that carries out phagocytosis by engulfing, absorbing, and digesting pathogens (such as bacteria and viruses) non-specifically.

What are antitoxins and what is their function?

Specific proteins produced by white blood cells that neutralise the toxic poisons (toxins) released by invading bacteria, preventing them from damaging body tissues.

How do aphids damage plants?

Aphids feed on sugary sap inside the phloem using piercing mouthparts, stunting plant growth and acting as vectors for plant viruses.

What are the three main factors tested when developing new drugs?

1. Toxicity: How safe the drug is and whether it causes harmful side effects. 2. Efficacy: Whether the drug actually works and treats the disease. 3. Dose: The optimum amount of the drug to give for maximum benefit with minimum toxicity.

What are the plant, fungus, and microorganism origins of aspirin, digitalis, and penicillin?

1. Aspirin (painkiller): Originates from willow bark. 2. Digitalis (heart drug): Originates from foxgloves. 3. Penicillin (antibiotic): Originates from Penicillium mould.

Why can antibiotics NOT kill viruses?

Antibiotics only kill bacteria by damaging their cell walls or cellular processes. Viruses reproduce inside host cells using the host's own cellular machinery, so antibiotics cannot target them without damaging human cells.

What happens during preclinical drug testing?

Drugs are tested in a laboratory using cells, tissues, and live animals to check for toxicity, efficacy, and dosage before any human trials begin.

What happens during clinical drug testing?

Drugs are tested on humans in clinical trials. First tested on healthy volunteers using low doses to check for safety/toxicity, then on patients suffering from the disease to find the optimum dose and check efficacy.

How does malaria spread and what causes it?

Caused by a protist pathogen. It is spread by a vector (the female Anopheles mosquito). The mosquito picks up the protist when feeding on an infected person's blood and injects it into an uninfected person's bloodstream during the next bite.

True or false: Painkillers destroy pathogens in our body?

False. Painkillers (like paracetamol or aspirin) only relieve the symptoms and pain of an infection; they do not kill the pathogens themselves.

What makes up vaccines?

Dead, inactive, or weakened strains of a pathogen containing specific antigens.

Where is salmonella bacteria found?

In food products (such as undercooked poultry, raw eggs, or unhygienically prepared food).

Name a viral disease in plants.

Tobacco Mosaic Virus (TMV), which causes mosaic discoloration on leaves and reduces photosynthesis.

Name a viral disease that can lead to severe complications like blindness or brain damage (encephalitis).

Measles (severe complications can include blindness, brain damage, or subacute sclerosing panencephalitis).

How can measles spread?

By breathing in droplets from sneezes and coughs of an infected person.

How can HIV spread?

Through sexual contact or the exchange of body fluids such as blood (e.g. sharing needles).

Define health.

A state of physical and mental well-being, not merely the absence of disease or infirmity.So being healthy isn't just about not having a sickness—it means you actually feel good in your body and mind, too.

How do bacteria make you ill?

By reproducing rapidly inside the body and releasing harmful toxins (poisons) that damage tissues and cells.

How do viruses make you ill?

By invading host cells, replicating rapidly inside them, and causing the host cells to burst and die.

State one method to prevent the spread of diseases.

Washing hands regularly, isolating infected individuals, using vaccination, or using contraception (condoms) for STDs.

How do pathogens spread?

Through direct contact, water, air (droplet infection), or vectors (such as insects).

How can bacterial diseases be treated?

Using antibiotics (such as penicillin) which kill infective bacteria inside the body.

State a long-term effect of gonorrhoea.

Long-term effects include pelvic inflammatory disease (PID), chronic lower abdominal pain, and infertility/ectopic pregnancies.

How can gonorrhoea be stopped/prevented?

Using barrier methods of contraception such as condoms and treating infected individuals with antibiotics.

Name a fungal disease in humans.

Athlete's foot (tinea pedis), which causes scaly, itchy rashes on the feet.

Which pathogen causes rose black spot?

A fungus (Diplocarpon rosae).

State a symptom for rose black spot.

Purple or black spots develop on leaves, which often turn yellow and drop off early, reducing photosynthesis.

Which pathogen causes malaria?

A protist (specifically Plasmodium species), spread by the female Anopheles mosquito vector.

What is a symptom of gonorrhoea?

Thick yellow or green discharge from the vagina or penis, and pain when urinating.

What is an antigen?

A specific protein molecule on the surface of a pathogen that is recognized as foreign by the immune system, triggering an immune response.

What are antibodies?

Y-shaped proteins produced by white blood cells (lymphocytes) that bind specifically to foreign antigens on pathogens to destroy or neutralise them.

How do white blood cells defend the body?

1. Phagocytosis (engulfing and digesting pathogens). 2. Producing specific antibodies to target antigens. 3. Producing antitoxins to neutralise bacterial poisons.

How does your stomach defend the body against pathogens?

By producing hydrochloric acid (HCl), which creates a low pH that kills most swallowed pathogens.

How does the trachea defend the body against pathogens?

The trachea and bronchi are lined with mucus to trap dust and pathogens, and ciliated cells beat rhythmically to sweep the mucus up to the throat to be swallowed and destroyed in stomach acid.

What living materials are used in preclinical testing?

Laboratory-grown cells, tissues, and live animals (such as mice or rats).

What is the difference between preclinical testing and clinical trial?

Preclinical testing is done in a laboratory using cells, tissues, and live animals before any human contact to check toxicity and efficacy. Clinical trials are carried out on living humans (first healthy volunteers, then patients) to monitor safety, dosage, and side effects.

How does a vaccine lead to immunity?

Dead/inactive pathogen injected $\rightarrow$ White blood cells produce specific antibodies $\rightarrow$ Memory cells remain in blood $\rightarrow$ Future exposure leads to a rapid secondary immune response.

How do the eyes defend against pathogens?

Produce tears containing enzymes (like lysozyme) which chemical-break down bacterial cell walls.

Why can painkillers not cure an infectious disease?

They only treat symptoms (such as pain and fever) and do not kill the pathogens causing the disease.