Spread of communicable diseases

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.

How Diseases Spread in Animals

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:

• Airborne (Droplets): Pathogens are expelled in mucus or saliva during coughing and inhaled by others (e.g., Tuberculosis, Covid-19).
• Waterborne: Ingesting water contaminated by infected faeces (e.g., Cholera).
• Foodborne: Eating undercooked or cross-contaminated food (e.g., Salmonella).

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 Anopheles mosquitoes act as vectors for malaria. They ingest the Plasmodium protist from an infected human's blood and inject it via saliva into a new host.

How Diseases Spread in Plants

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.

Pathogen Numbers and Infectious Dose

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.

• Diseases like E. coli have a very low infectious dose (10–100 cells), making them highly contagious.
• Cholera (Vibrio cholerae) requires a very high infectious dose (millions of cells) to survive stomach acid, so it usually only spreads when water is heavily contaminated.

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.

Worked Example: Infection Potential

A patient infected with cholera excretes $10^{11}$ bacteria per day into a local water supply. If the infectious dose required to cause cholera is $10^5$ bacteria, how many people could theoretically be infected by this single patient?

Step 1: State the formula for potential infections.

$$\text{Potential Infections} = \frac{\text{Total pathogens excreted}}{\text{Infectious dose}}$$

Step 2: Substitute the values into the equation.

$$\text{Potential Infections} = \frac{10^{11}}{10^5}$$

Step 3: Calculate the final answer using exponent rules (subtract the powers).

$$\text{Potential Infections} = 10^6 \text{ people (1 million people)}$$

Analyzing Disease Data

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.

Worked Example: Incidence Rate

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.

$$\text{Incidence Rate} = \left( \frac{\text{New cases}}{\text{Total population}} \right) \times 100{,}000$$

Step 2: Substitute the known values.

$$\text{Incidence Rate} = \left( \frac{15}{25{,}000} \right) \times 100{,}000$$

Step 3: Calculate the final answer.

$$\text{Incidence Rate} = 0.0006 \times 100{,}000 = 60 \text{ cases per 100,000 people}$$

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.