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Viruses reproduce inside host cells, while bacteria reproduce rapidly and produce toxins that damage tissues. Free notes and 13 questions answered.
How Pathogens Cause Disease is part of Communicable diseases in GCSE AQA Higher Biology.
4 min
Reading time
17
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
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Viruses reproduce inside host cells, while bacteria reproduce rapidly and produce toxins that damage tissues. Free notes and 13 questions answered.
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Use this lesson to stay inside Communicable diseases while connecting the detail back to the wider Infection and response topic.
Read the core explanations from How Pathogens Cause Disease before testing yourself from memory.
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.
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?
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.
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.
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.
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.
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.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Viruses cause disease by living and reproducing inside host cells, forcing the cell to make copies of the virus instead of doing its normal job. This damages or destroys the cell, and when the cell bursts open, hundreds of new viruses are released to infect more cells.
Bacteria cause disease by reproducing rapidly by binary fission and producing toxins, poisonous substances that damage tissues and cause illness symptoms. Most bacterial damage comes from the toxins they release rather than from the bacteria destroying cells directly.
Viruses do not produce toxins; it is bacteria that release toxins. Viruses damage the body by living and reproducing inside cells, which damages or destroys them, while most damage from bacteria comes from the toxins they release.
A host cell is a living cell that provides nutrients and reproduction machinery for viruses to multiply inside. The virus forces the host cell to make copies of the virus, which damages or destroys the cell.
Bacteria reproduce by binary fission, doubling their numbers every 20-30 minutes under ideal conditions. If one bacterium divides every 20 minutes, after 2 hours there are 120 ÷ 20 = 6 divisions, so 1 × 2 to the power 6 = 64 bacteria.
Bacteria mostly stay outside cells and cause damage through toxins, often giving localised symptoms such as diarrhoea in food poisoning. Viruses live and reproduce inside cells and damage them directly, tending to cause more general symptoms such as inflammation and fever as cells die.
A toxin is a poisonous substance produced by bacteria that damages tissues and causes illness symptoms. For example, food poisoning bacteria produce toxins that damage the intestinal lining, causing diarrhoea and stomach cramps. Viruses do not produce toxins.
A single infected cell can produce 100-1000 new virus particles before it dies. When the cell bursts open, the new viruses are released to infect more cells, though they need time to find and enter new host cells.
A single bacterium that divides every 20 minutes becomes 64 bacteria after 2 hours. There are 120 ÷ 20 = 6 divisions, and the number is 1 × 2⁶ = 64, showing how quickly bacterial infections can make people ill.
Viral infections cause fever and more general symptoms because viruses destroy cells throughout the body. When cells die, they release chemicals that trigger inflammation, fever and the general feeling of being unwell.
Food poisoning causes diarrhoea and stomach cramps because food poisoning bacteria produce toxins that damage the intestinal lining. Bacterial infections often cause localised symptoms like this because the toxins damage specific tissues.
Bacteria mostly stay outside cells, whereas viruses live and reproduce inside cells. Most bacterial damage comes from the toxins they release, not from the bacteria themselves destroying cells, so saying bacteria destroy cells directly is a common mistake.
Bacterial infections can cause illness quickly because bacteria reproduce rapidly by binary fission, doubling every 20-30 minutes under ideal conditions. This exponential growth means toxins from millions of bacteria can overwhelm the immune system.