How Pathogens Cause Disease Revision Notes | GCSE AQA Higher Biology

1. What Makes You Sick?

You wake up with a sore throat, fever, and feel terrible. But what's actually happening inside your body? The culprits are pathogens - microscopic invaders that have found their way into your system and are now causing chaos.

There are two main types of pathogens that make us ill: bacteria and viruses. They might both be tiny, but they cause disease in completely different ways. Understanding these differences is crucial for your exams and explains why some illnesses need antibiotics while others don't.

2. Bacterial Invasion - The Toxin Makers

Picture bacteria as tiny factories that have invaded your body. Once inside, they don't just sit there - they get to work immediately. Bacteria reproduce rapidly by binary fission, doubling their numbers every 20-30 minutes under ideal conditions.

Here's where it gets nasty. Many bacteria produce toxins - poisonous chemicals that damage your tissues directly. These toxins are like molecular weapons that attack your cells, causing the symptoms you feel when you're ill.

Trap: Many students think bacteria directly attack cells like viruses do. Actually, most bacterial damage comes from the toxins they release, not from the bacteria themselves destroying cells.

Worked example: How Bacterial Numbers Explode

A single bacterium enters your throat. If it divides every 20 minutes, how many bacteria will there be after 2 hours?

1. 2 hours = 120 minutes 120 ÷ 20 = 6 divisions — Each division doubles the population
2. Number = $1 \times 2^6$ — Start with 1, double 6 times
3. $2^6 = 64$ bacteria — From 1 to 64 in just 2 hours!

Answer: 64 bacteria after 2 hours

This exponential growth explains why bacterial infections can make you feel ill so quickly. The toxins from millions of bacteria overwhelm your immune system.

3. Viral Hijacking - The Cell Destroyers

Viruses work completely differently from bacteria. Think of a virus as a microscopic pirate that hijacks your cells. Viruses cannot reproduce on their own - they need to get inside your cells and use your cellular machinery.

Once inside a host cell, the virus takes control. It forces the cell to make copies of the virus instead of doing its normal job. This process damages or destroys the host cell, and when the cell bursts open, hundreds of new viruses are released to infect more cells.

The key difference is that viruses live and reproduce inside cells, causing cell damage directly. Bacteria mostly stay outside cells and cause damage through toxins.

4. The Speed of Infection

Both bacteria and viruses can reproduce rapidly inside the body, but they do it differently. Bacteria reproduce by binary fission - one cell splits into two identical cells. Under perfect conditions, this happens every 20 minutes.

Viruses reproduce even faster once inside a cell. A single infected cell can produce 100-1000 new virus particles before it dies. However, viruses need time to find and enter new host cells, which can slow down the overall infection rate.

5. Why You Feel Ill - The Damage Explained

The symptoms you experience during an infection aren't random - they're direct results of how pathogens damage your body. Understanding this helps explain why different infections cause different symptoms.

Bacterial infections often cause localised symptoms because toxins damage specific tissues. For example, food poisoning bacteria produce toxins that damage your intestinal lining, causing diarrhoea and stomach cramps.

Viral infections tend to cause more general symptoms because they destroy cells throughout your body. When cells die, they release chemicals that trigger inflammation, fever, and the general feeling of being unwell.

6. Exam Technique - Getting Full Marks

Exam questions about pathogens often test whether you understand the different mechanisms. The key is being specific about HOW each type causes disease, not just stating that they do.

Common exam mistakes include saying viruses produce toxins (they don't) or that bacteria destroy cells directly (most damage comes from toxins). Be precise with your terminology.