Every time someone sneezes on a crowded bus, millions of microscopic invaders are launched into the air. These microscopic invaders are pathogens—microorganisms that cause communicable diseases.
Pathogens can be grouped into four main types: viruses, bacteria, fungi, and protists. Once they infect an organism, they must find a way to transmit themselves to a new host to survive.
This spread usually happens through horizontal transmission (from one individual to another in the same generation, like person-to-person). Less commonly, it occurs via vertical transmission (from parent to offspring, such as during birth or via seeds in plants). The exam board categorises horizontal spread into four main routes: airborne droplets, waterborne transmission, direct contact, and vectors.
The speed at which viral and bacterial diseases spread is heavily influenced by human factors. High population density increases the frequency of contact between individuals, allowing airborne and direct contact diseases to spread rapidly. Furthermore, modern global travel means an infected person can cross the world in hours, carrying a pathogen to new, susceptible populations.
This rapid spread is often facilitated by a pathogen's incubation period—the time between the initial infection and the appearance of the first symptoms. During this period, an infected person may feel completely healthy, allowing them to travel and unknowingly transmit the disease to others.
Viruses cause damage by invading host cells and hijacking their machinery to replicate until the cell bursts (cell lysis).
In animals, viral transmission is highly varied:
In plants, the Tobacco Mosaic Virus (TMV) primarily spreads via direct contact when infected leaves rub against healthy ones. It can also spread indirectly via contaminated human hands or gardening tools. To enter the plant's cytoplasm, the virus requires a physical wound that breaks the waxy cuticle. Some plant vectors, like aphids, bypass this by biting into the stem and injecting the virus directly into the plant's phloem. Once inside, the virus moves systemically through the plant via the phloem and plasmodesmata.
Bacteria typically cause illness by producing harmful toxins that damage host tissues. They multiply rapidly through binary fission.
Animal bacterial infections spread through multiple routes:
In plants, Agrobacterium tumefaciens causes crown gall disease. It spreads via contaminated soil, water splash, or contaminated tools. The bacteria enter through a wound and use a special Ti plasmid to transfer T-DNA into the plant's genome. This triggers rapid cell division, forming large tumours (galls) that restrict the flow of water and nutrients.
You can calculate how quickly a bacterial infection like Salmonella spreads using the population growth formula:
Where:
Worked Example:
Calculate the final population of a single bacterium after 4 generations.
Step 1: Identify the known values.
Step 2: Substitute the values into the equation.
Step 3: Calculate the final answer.
PAG 7 Calculation: In practical experiments involving antibiotics or antiseptics, you may need to calculate the area of a bacterial colony or the clear zone (zone of inhibition) around an antibiotic disc. Assuming the zone is roughly circular, you use the formula for the area of a circle:
Where is the radius of the colony or clear zone.
Multicellular fungi are made of thread-like filaments called hyphae, which weave together to form a mycelium. While hyphae penetrate host tissues and secrete extracellular enzymes to digest them, the actual spread to new hosts relies on spores.
Fungal pathogens use highly adapted asexual spores to travel long distances:
Protists are eukaryotic, mostly single-celled parasites. Their transmission heavily relies on intermediate carrier organisms.
Malaria is caused by the Plasmodium protist. The vector is the female Anopheles mosquito. When an infected mosquito bites a human, it injects the protist via its saliva into the bloodstream. The pathogen multiplies asexually in the human liver, then enters red blood cells, causing them to burst and triggering recurrent fever.
In plants, Potato Blight is caused by the protist-like Phytophthora infestans. It thrives in damp conditions, spreading via wind and waterborne spores.
Understanding the physical mechanisms of spread allows scientists to develop specific prevention strategies.
| Feature | Airborne Transmission | Waterborne Transmission |
|---|---|---|
| Mechanism | Pathogens travel suspended in air (heavy droplets settle quickly, tiny aerosols travel further, wind carries spores). | Pathogens travel suspended in water (ingestion of contaminated water or rain splash on leaves). |
| Plant Defences | None specifically against wind, but thick cell walls resist entry. | The hydrophobic waxy cuticle causes contaminated water to roll off the leaf, preventing entry. |
| Prevention Strategy | Improved ventilation, wearing masks, and social distancing (animals); removing infected fallen leaves (plants). | Boiling water, chlorination, and improved sewage systems (animals); watering the soil rather than the foliage (plants). |
Students often state that the mosquito causes malaria. The mosquito is actually the vector; the disease is caused by the Plasmodium pathogen.
When answering questions about the spread of HIV, you must specifically mention the 'exchange of body fluids' such as sharing needles or unprotected sexual contact to secure the mark.
In 'Explain' questions about disease prevention, always link your answer causally to the transmission method (e.g., 'covering your mouth prevents the release of airborne droplets containing the virus').
Do not confuse waterborne transmission (the physical medium moving the pathogen) with water availability (the physiological requirement of a plant).
For PAG 7 calculations, remember that the clear zone around an antibiotic disc is calculated using the area of a circle. Ensure you halve the diameter to find the radius before using the formula!
Pathogen
A microorganism (such as a virus, bacterium, fungus, or protist) that causes communicable disease.
Communicable disease
An infectious disease that can be passed from one organism to another.
Viruses
Small infectious agents that can only replicate inside the living cells of an organism, often causing damage via cell lysis.
Bacteria
Single-celled prokaryotic organisms, some of which cause illness by producing harmful toxins.
Fungi
Eukaryotic organisms that can be single-celled or multicellular, using hyphae to penetrate tissues and spread via spores.
Protists
Eukaryotic, mostly single-celled organisms, many of which are parasites that rely on vectors for transmission.
Incubation period
The time between initial infection by a pathogen and the appearance of the first symptoms.
Horizontal transmission
The spread of a pathogen between members of the same species who are not in a parent-offspring relationship (e.g., person-to-person).
Vertical transmission
The spread of a pathogen from a parent to its offspring, such as during birth, breastfeeding, or via seeds.
Airborne droplets
Small particles of moisture containing pathogens released during coughing or sneezing that can be inhaled by others.
Waterborne transmission
The spread of pathogens via ingestion of contaminated water or via water splashes on plant leaves.
Direct contact
Transmission of a pathogen through physical touch or the exchange of body fluids between infected and susceptible hosts.
Vector
An organism, often an insect, that carries a pathogen from one host to another without being affected by the disease itself.
Cell lysis
The bursting or rupturing of a host cell membrane, often caused when viruses replicate rapidly inside.
Binary fission
A type of asexual reproduction where a single-celled organism divides into two identical cells, allowing bacteria to multiply rapidly.
Plasmodesmata
Microscopic channels through plant cell walls that allow transport and communication, which viruses use to spread internally.
Ti plasmid
A tumour-inducing circular piece of DNA found in Agrobacterium tumefaciens, used to transfer T-DNA into a plant's genome.
T-DNA
Transfer DNA; the specific segment of the Ti plasmid that is integrated into the host plant's genome, causing crown gall disease.
Hyphae
Thread-like filaments in fungi that penetrate host tissues and secrete enzymes to absorb nutrients.
Mycelium
A network of thread-like filaments (hyphae) that forms the vegetative part of a fungus.
Asexual spores
Microscopic reproductive cells produced by fungi and some protists, adapted for mass dispersal by wind or water.
Fomite
An inanimate object, such as a towel or changing room floor, that serves as a vehicle for transferring pathogens.
Put your knowledge into practice — try past paper questions for Biology B
Pathogen
A microorganism (such as a virus, bacterium, fungus, or protist) that causes communicable disease.
Communicable disease
An infectious disease that can be passed from one organism to another.
Viruses
Small infectious agents that can only replicate inside the living cells of an organism, often causing damage via cell lysis.
Bacteria
Single-celled prokaryotic organisms, some of which cause illness by producing harmful toxins.
Fungi
Eukaryotic organisms that can be single-celled or multicellular, using hyphae to penetrate tissues and spread via spores.
Protists
Eukaryotic, mostly single-celled organisms, many of which are parasites that rely on vectors for transmission.
Incubation period
The time between initial infection by a pathogen and the appearance of the first symptoms.
Horizontal transmission
The spread of a pathogen between members of the same species who are not in a parent-offspring relationship (e.g., person-to-person).
Vertical transmission
The spread of a pathogen from a parent to its offspring, such as during birth, breastfeeding, or via seeds.
Airborne droplets
Small particles of moisture containing pathogens released during coughing or sneezing that can be inhaled by others.
Waterborne transmission
The spread of pathogens via ingestion of contaminated water or via water splashes on plant leaves.
Direct contact
Transmission of a pathogen through physical touch or the exchange of body fluids between infected and susceptible hosts.
Vector
An organism, often an insect, that carries a pathogen from one host to another without being affected by the disease itself.
Cell lysis
The bursting or rupturing of a host cell membrane, often caused when viruses replicate rapidly inside.
Binary fission
A type of asexual reproduction where a single-celled organism divides into two identical cells, allowing bacteria to multiply rapidly.
Plasmodesmata
Microscopic channels through plant cell walls that allow transport and communication, which viruses use to spread internally.
Ti plasmid
A tumour-inducing circular piece of DNA found in Agrobacterium tumefaciens, used to transfer T-DNA into a plant's genome.
T-DNA
Transfer DNA; the specific segment of the Ti plasmid that is integrated into the host plant's genome, causing crown gall disease.
Hyphae
Thread-like filaments in fungi that penetrate host tissues and secrete enzymes to absorb nutrients.
Mycelium
A network of thread-like filaments (hyphae) that forms the vegetative part of a fungus.
Asexual spores
Microscopic reproductive cells produced by fungi and some protists, adapted for mass dispersal by wind or water.
Fomite
An inanimate object, such as a towel or changing room floor, that serves as a vehicle for transferring pathogens.