Infection and response — section overview

B3 covers how pathogens cause disease, how the body defends itself, and how we use vaccinations and drugs to combat illness.

Types of pathogen

Type	Examples	Treatment
Bacteria	TB, cholera, Salmonella	Antibiotics
Virus	Measles, HIV, influenza, COVID-19	Antivirals (limited); vaccines
Fungi	Athlete's foot, rose black spot	Antifungals
Protist	Malaria (Plasmodium)	Antimalarials

Key distinction: viruses reproduce inside host cells; bacteria reproduce independently. This is why antibiotics kill bacteria but not viruses.

Spreading disease

• Direct contact: skin, sexual contact (e.g. HIV, gonorrhoea)
• Droplets: sneezing/coughing (influenza, COVID-19, measles)
• Contaminated water/food: cholera, Salmonella
• Vectors: mosquitoes carry Plasmodium (malaria); aphids carry plant viruses

The body's defences

Non-specific (first and second line):

1. Skin — physical barrier; sebum (oily secretion) is slightly acidic
2. Mucus + cilia in airways — trap and sweep out pathogens
3. Stomach acid (pH 2) — kills most ingested pathogens
4. Inflammation — increased blood flow to infected area

Specific (third line — immune system):

• Phagocytes (neutrophils, macrophages) — engulf and digest pathogens (phagocytosis)
• Lymphocytes — B-cells produce antibodies; T-cells destroy infected cells
• Antibodies — specific proteins that bind to antigens on pathogens, marking them for destruction
• Memory cells — remain in the blood; enable faster, stronger response on re-infection

Vaccination

Vaccination introduces a dead or weakened pathogen (or antigen/mRNA) so the immune system produces memory cells without causing disease. On future exposure, the rapid response prevents illness.

Herd immunity: if enough of the population is vaccinated (~95% for measles), even unvaccinated individuals are protected because the pathogen cannot spread.

Drug development

Stages: discovery → pre-clinical trials (cell cultures/animals) → Phase I (small group of healthy volunteers, safety) → Phase II (small patient group, effectiveness/dosage) → Phase III (large randomised controlled trial — placebo/double-blind) → regulatory approval.

Antibiotic resistance: arises through natural selection — bacteria with resistance mutations survive and reproduce. Caused by: overuse of antibiotics, not completing courses, use in farming.

Common exam mistakes in B3

1. Antibiotics kill viruses — FALSE: antibiotics target bacterial cell walls/proteins
2. Vaccines give you the disease — FALSE: dead/weakened forms or just antigens are used
3. Antigens vs antibodies — antigens are on the pathogen surface; antibodies are produced by B-lymphocytes