Disease and Epidemiology
Explore the types of pathogens that cause disease, understand modes of transmission, learn about the basic reproduction number (R0), and examine public health strategies for disease control.
Types of Pathogens
A pathogen is a microorganism that can cause disease in a host. Infectious diseases are caused by four main types of pathogens. Each type has different structures, replication methods, and requires different treatment strategies.
Four Major Pathogen Types
Bacteria
- • Prokaryotic, single-celled organisms
- • Reproduce by binary fission
- • Treated with antibiotics
- • Examples: E. coli, Mycobacterium tuberculosis
Viruses
- • Non-cellular, require host cell to replicate
- • DNA or RNA genome in protein coat
- • Treated with antivirals (antibiotics do not work)
- • Examples: Influenza, SARS-CoV-2, HIV
Fungi
- • Eukaryotic, can be unicellular or multicellular
- • Reproduce by spores
- • Treated with antifungals
- • Examples: Candida, Aspergillus, tinea
Parasites (Protists & Helminths)
- • Eukaryotic organisms that live on/in a host
- • Complex life cycles, often with vectors
- • Treated with antiparasitics
- • Examples: Plasmodium (malaria), tapeworms
Key point: Prions are an additional category -- misfolded proteins that cause neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease). They contain no nucleic acid and cannot be treated with antibiotics, antivirals, or antifungals.
Transmission and R0
Disease transmission refers to how pathogens spread between hosts. The basic reproduction number (R0) -- pronounced "R-naught" -- is a key epidemiological measure that estimates the average number of people one infected person will spread the disease to in a fully susceptible population.
Modes of Transmission
Direct Contact
Person-to-person: droplets, bodily fluids, skin contact (e.g., flu, HIV)
Indirect (Vehicle)
Via contaminated food, water, surfaces, air (e.g., cholera, COVID-19)
Vector-borne
Via an organism carrier (mosquito, tick, flea) (e.g., malaria, dengue)
Understanding R0
R0 < 1: Each infected person infects fewer than one other person on average. The outbreak will decline and die out.
R0 = 1: The disease is endemic -- stable, neither growing nor declining.
R0 > 1: The disease spreads exponentially. The higher the R0, the harder it is to control (e.g., measles R0 ≈ 12-18, COVID-19 original strain R0 ≈ 2-3).
Public Health Strategies
Epidemiology is the study of how diseases spread in populations and how to control them. Public health measures aim to reduce R0 below 1 through a combination of strategies targeting different links in the chain of infection.
Disease Control Measures
Vaccination
Herd immunity achieved when enough of the population is immune (threshold = 1 - 1/R0)
Quarantine & Isolation
Separating exposed (quarantine) or infected (isolation) individuals to break transmission chains
Hygiene & Sanitation
Hand washing, clean water, food safety, waste management to reduce pathogen exposure
Herd Immunity Threshold
- Formula: Herd immunity threshold = 1 - 1/R0
- Measles (R0 ≈ 15): 1 - 1/15 = 93% of the population must be immune.
- Seasonal flu (R0 ≈ 2): 1 - 1/2 = 50% of the population must be immune.
- Implication: Diseases with higher R0 require a larger proportion of the population to be vaccinated to achieve herd immunity.
Key Vocabulary
Pathogen
A disease-causing microorganism, including bacteria, viruses, fungi, protists, and prions, that can infect a host and cause illness.
R0 (Basic Reproduction Number)
The average number of secondary infections produced by one infected individual in a completely susceptible population. A key measure of disease transmissibility.
Herd Immunity
Indirect protection from infectious disease that occurs when a sufficient proportion of the population is immune, reducing the likelihood of infection for susceptible individuals.
Epidemiology
The branch of science that studies the distribution, causes, and control of diseases in populations, informing public health policy and disease prevention strategies.
Worked Examples
A disease has an R0 of 4. Calculate the herd immunity threshold.
Step 1: Use the formula: Herd immunity threshold = 1 - 1/R0
Step 2: Threshold = 1 - 1/4 = 1 - 0.25 = 0.75
Answer: 75% of the population must be immune (through vaccination or prior infection) to achieve herd immunity and stop the spread of this disease.
Explain why antibiotics are ineffective against viral infections.
Step 1: Antibiotics target structures and processes specific to bacteria, such as cell wall synthesis (penicillin), protein synthesis on 70S ribosomes, or DNA replication enzymes.
Step 2: Viruses lack these bacterial structures. They have no cell wall, no ribosomes of their own, and use the host cell's machinery to replicate.
Answer: Since viruses replicate inside host cells using host machinery, antibiotics have no target to act upon. Antiviral drugs that target viral-specific processes (e.g., viral polymerase, protease) are needed instead.
Describe how public health measures during a pandemic aim to reduce the effective reproduction number below 1.
Step 1: Social distancing and lockdowns reduce contact between susceptible and infected individuals, lowering the transmission rate.
Step 2: Vaccination reduces the number of susceptible people in the population.
Answer: Together with hygiene measures (hand washing, masks), contact tracing, and isolation of cases, these interventions reduce the effective reproduction number (Re) below 1. When Re < 1, each infected person infects fewer than one other person on average, and the epidemic declines.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Which type of pathogen requires a host cell to replicate?
Question 2
If a disease has R0 = 5, the herd immunity threshold is:
Question 3
Malaria is transmitted by:
Question 4
When R0 is greater than 1, the number of infections in a population will:
Question 5
Antibiotic resistance arises primarily through:
Key Concepts Summary
- ●Pathogens include bacteria, viruses, fungi, and parasites, each requiring different treatment approaches.
- ●Diseases spread by direct contact, indirect (vehicle) transmission, or vectors (e.g., mosquitoes).
- ●R0 measures transmissibility: R0 > 1 means the disease spreads; R0 < 1 means it declines.
- ●Herd immunity threshold = 1 - 1/R0. Higher R0 requires greater vaccination coverage.
- ●Public health strategies (vaccination, quarantine, hygiene, contact tracing) aim to reduce Re below 1 to control outbreaks.