OCR GCSE Biology A (J247) — Global challenges
Every time you sneeze, you could be launching millions of microscopic invaders into the air. These invaders are called pathogens, which are microorganisms that cause disease in another organism (the host). Diseases caused by pathogens are known as communicable diseases because they can spread between individuals.
There are four main types of pathogen, each with a different causal mechanism for how they infect and damage the host:
After a pathogen enters the body, there is an incubation period. This is the time between the initial infection and the first appearance of symptoms, during which the pathogen replicates rapidly.
You can catch a cold without ever touching the person who gave it to you. In animals and humans, transmission occurs through several distinct pathways.
Direct contact involves physical transfer, such as skin-to-skin contact, sexual transmission (e.g., HIV), or placental transfer from mother to foetus. Indirect transmission relies on environmental factors:
Some diseases rely on a vector to spread. A vector is an organism that carries a pathogen from one host to another without being affected by the disease itself. For example, female mosquitoes act as vectors for malaria. They ingest the protist from an infected human's blood and inject it via saliva into a new host.
Plants cannot run away from infections, making them highly vulnerable to environmental spread. Fungal diseases like Ash dieback are heavily reliant on wind (airborne) transmission. The fungus releases lightweight spores that can travel huge distances—studies show some spores can travel from mainland Europe to the UK, with about 10 in every 1 billion spores successfully crossing the sea.
Water is another major transmission route. Diseases like Rose black spot spread via rain splashing from leaf to leaf, while Potato blight produces swimming zoospores that move through water films into the soil. Pathogens like Crown gall disease can survive entirely in the soil and infect roots.
Plants also suffer from direct contact. The Tobacco Mosaic Virus (TMV) spreads when infected leaves touch healthy ones, or via contaminated agricultural tools. Aphids act as insect vectors, transmitting viruses while feeding on plant sap. These infections often destroy chlorophyll, leading to chlorosis (yellowing of the leaves).
To fight back, plants have physical defenses like a waxy cuticle and callose production (which temporarily blocks pathogen entry), alongside chemical defenses. Farmers use crop rotation—planting different species in successive years—to break the pathogen lifecycle in the soil.
Drinking a single bacterial cell rarely makes you sick, but gulping down a million might. The severity and likelihood of an outbreak depend heavily on the pathogen load (the concentration of the pathogen in the environment).
To cause disease, a specific number of pathogens must enter the body. This is known as the infectious dose.
Because testing water for every specific disease is difficult, scientists look for indicator bacteria to assess the risk. If a sample shows more than 100 indicator bacteria per 100 ml, there is a direct correlation with an increased risk of disease.
A patient infected with cholera excretes bacteria per day into a local water supply. If the infectious dose required to cause cholera is bacteria, how many people could theoretically be infected by this single patient?
Step 1: State the formula for potential infections.
Step 2: Substitute the values into the equation.
Step 3: Calculate the final answer using exponent rules (subtract the powers).
Epidemiologists act like detectives, using data to track and predict outbreaks. When analysing data, they look at two main metrics: the incidence of disease (the rate of new cases) and prevalence (the total number of existing cases).
Areas with high population density usually face higher infection rates due to increased contact rates. However, a high rate of herd immunity (where many individuals are resistant via vaccination or prior infection) drastically reduces transmission.
In a town with a population of 25,000, there were 15 new cases of tuberculosis detected in one year. Calculate the incidence rate per 100,000 people.
Step 1: State the formula for incidence rate.
Step 2: Substitute the known values.
Step 3: Calculate the final answer.
When looking at scatter graphs of disease data, you might see a strong correlation (e.g., cases rise as population density rises). However, you must remember that correlation does not prove causation without a biological mechanism. Examiners will also expect you to spot patterns such as a time lag—the delay between pathogen detection in the environment and the peak of disease symptoms in the population.
Students often confuse 'incidence' (the rate of new cases) with 'prevalence' (the total existing cases) — read graph axes carefully to see which metric is being shown.
In 6-mark questions about transmission, always name the specific biological mechanism (e.g., 'viruses cause cell lysis' or 'bacteria replicate by binary fission') rather than just saying they 'multiply'.
When asked to 'analyse' a scatter graph, you must quote specific numbers from both axes and identify the trend (e.g., 'as pathogen concentration rises above X, cases increase to Y').
Remember that correlation does not prove causation; if a graph shows high population density correlates with high infection rates, you must explain the biological reason (increased contact rates) to get full marks.
Ensure you clearly distinguish between the vector (the Anopheles mosquito) and the pathogen (the Plasmodium protist) when discussing malaria.
Pathogen
A microorganism that causes disease in another organism (plant or animal).
Communicable disease
A disease caused by a pathogen that can be spread between individuals.
Binary fission
The specific method of asexual reproduction used by bacteria, leading to exponential growth.
Exponential growth
Rapid reproduction where numbers double at regular intervals (e.g., ).
Lysis
The bursting of a host cell after viral replication, releasing new virus particles into the organism.
Incubation period
The time between initial infection by a pathogen and the first appearance of symptoms.
Vector
An organism (usually an insect) that carries a pathogen from one host to another without being affected by the disease itself.
Chlorosis
The yellowing of plant leaves due to a lack of chlorophyll, often a symptom of disease.
Callose production
A plant defense mechanism involving the temporary deposition of callose to block pathogen entry.
Crop rotation
The practice of growing different crops in succession on the same land to break the lifecycle of soil-borne pathogens.
Pathogen load
The total amount or concentration of a pathogen (e.g., bacterial count per ml of water) in a sample.
Infectious dose
The minimum number of pathogens required to successfully establish an infection in a host.
Incidence of disease
The rate at which new cases of a disease occur in a population over a specific period.
Prevalence
The total number of existing cases in a population at a specific point in time.
Population density
The number of individuals per unit area (e.g., people per square kilometre or plants per square metre).
Herd immunity
Protection from a disease in a population where a high proportion of individuals are immune, reducing the probability of transmission.
Correlation
A relationship between two variables, which does not necessarily prove that one causes the other.
Time lag
The delay between pathogen detection in the environment and the peak of disease symptoms in the population.
Put your knowledge into practice — try past paper questions for Biology A
Pathogen
A microorganism that causes disease in another organism (plant or animal).
Communicable disease
A disease caused by a pathogen that can be spread between individuals.
Binary fission
The specific method of asexual reproduction used by bacteria, leading to exponential growth.
Exponential growth
Rapid reproduction where numbers double at regular intervals (e.g., ).
Lysis
The bursting of a host cell after viral replication, releasing new virus particles into the organism.
Incubation period
The time between initial infection by a pathogen and the first appearance of symptoms.
Vector
An organism (usually an insect) that carries a pathogen from one host to another without being affected by the disease itself.
Chlorosis
The yellowing of plant leaves due to a lack of chlorophyll, often a symptom of disease.
Callose production
A plant defense mechanism involving the temporary deposition of callose to block pathogen entry.
Crop rotation
The practice of growing different crops in succession on the same land to break the lifecycle of soil-borne pathogens.
Pathogen load
The total amount or concentration of a pathogen (e.g., bacterial count per ml of water) in a sample.
Infectious dose
The minimum number of pathogens required to successfully establish an infection in a host.
Incidence of disease
The rate at which new cases of a disease occur in a population over a specific period.
Prevalence
The total number of existing cases in a population at a specific point in time.
Population density
The number of individuals per unit area (e.g., people per square kilometre or plants per square metre).
Herd immunity
Protection from a disease in a population where a high proportion of individuals are immune, reducing the probability of transmission.
Correlation
A relationship between two variables, which does not necessarily prove that one causes the other.
Time lag
The delay between pathogen detection in the environment and the peak of disease symptoms in the population.
