3.7.1Inquiry Process
Pathogens — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 150 of 160
Part 1 of 5Types of Pathogen
Bacteria
- Prokaryotic cells that damage tissue or release toxins; examples include Mycobacterium tuberculosis,
- which causes tuberculosis,
- and Salmonella,
- which causes food poisoning.
Viruses
- Non-cellular particles that must enter host cells to replicate;
- examples include influenza, HIV and the SARS-CoV-2 virus.
Fungi
- Eukaryotic organisms that can infect the skin or lungs, such as Candida albicans causing thrush.
Protists
- Eukaryotic single-celled parasites such as Plasmodium, which causes malaria and is spread by mosquitoes.
Other parasites
- Some worms and similar organisms live in or on a host and harm it, adding to the group of agents that cause disease.
| Group | Cellular? | Treatable with antibiotics? |
|---|---|---|
| Bacteria | Yes, prokaryotic | Yes |
| Viruses | No | No |
| Fungi | Yes, eukaryotic | No, use antifungals |
| Protists | Yes, eukaryotic | No, use antiprotozoal drugs |
Part 2 of 5Modes of Transmission
Modes of Transmission
Direct contact
- Pathogens pass through touch or body fluids, as in skin infections and sexually transmitted infections.
Droplets and aerosols
- Coughs and sneezes release tiny drops containing viruses or bacteria that others inhale.
Food and water
- Faecal-oral spread occurs when contaminated food or drinking water carries pathogens such as cholera bacteria.
Vectors
- Organisms such as mosquitoes carry pathogens from one host to another without necessarily being ill themselves.
Plant pathogens
- Plants are infected too, by fungi, bacteria and viruses spread by wind, soil, water and insects, which can reduce crop yields.
Part 3 of 5Course of an Infection
1
The pathogen enters through a break in the skin, a mucous membrane, or by swallowing or inhaling it.
2
It multiplies in the host, using the host’s resources, during the incubation period before symptoms appear.
3
It damages cells directly or releases toxins, producing the signs and symptoms of disease.
4
The immune system responds and, in many cases, clears the infection and leaves memory cells.
Part 4 of 5Worked Example
Question
- A bacterium divides once every 20 minutes.
- Starting with 100 cells, calculate the number after 3 hours if conditions stay ideal.
- 3 hours = 180 minutes, so the number of divisions = 180 ÷ 20 = 9.
- Number of cells = 100 × 2⁹ = 100 × 512.
- 100 × 512 = 51 200.
Answer About 51 200 bacteria, showing how fast infections can grow before the immune response begins.
Part 5 of 5Exam Checklist & Tips
3.7.1 – Key Points for the Exam
- Pathogen An organism or virus that causes disease.
- Vector An organism that carries a pathogen between hosts.
- Incubation period Time between infection and the first symptoms.
- Toxin A poisonous substance produced by a pathogen that harms the host.
Common Mistakes
- Treating viruses with antibiotics, which only work on bacteria.
- Describing viruses as cells; they are non-cellular.
- Saying every microorganism causes disease; most are harmless or helpful.
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