Main disease-causing organisms (pathogens) in animals and plants
pathogenMicroorganism that causes disease. are disease-causing virusAn ultramicroscopic infectious non-cellular organism that can replicate inside the cells of living hosts, with negative consequences., bacteriaSingle-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium., fungusA large group of eukaryotic organisms that contain single celled yeasts, moulds and mushrooms. or protistLarge group of eukaryotic organisms that can be single-celled or multicellular. Some protists cause disease, eg malaria.. All organisms are affected by pathogens. Even bacteria can be infected by certain types of virus.
There are four main types of pathogen.
| Pathogen | Width (渭m) | Example in animals | Example in plants |
| Viruses | 0.02 - 0.3 | HIV potentially leading to AIDS | Tobacco mosaic virus |
| Bacteria | 1 - 5 | Salmonella | Agrobacterium |
| Protists | 1 + | Malaria | Downy mildew |
| Fungi | 50 + | Athlete's foot | Black rose spot |
| Pathogen | Viruses |
|---|---|
| Width (渭m) | 0.02 - 0.3 |
| Example in animals | HIV potentially leading to AIDS |
| Example in plants | Tobacco mosaic virus |
| Pathogen | Bacteria |
|---|---|
| Width (渭m) | 1 - 5 |
| Example in animals | Salmonella |
| Example in plants | Agrobacterium |
| Pathogen | Protists |
|---|---|
| Width (渭m) | 1 + |
| Example in animals | Malaria |
| Example in plants | Downy mildew |
| Pathogen | Fungi |
|---|---|
| Width (渭m) | 50 + |
| Example in animals | Athlete's foot |
| Example in plants | Black rose spot |
For comparison a human hair is 17 - 18 渭m across.
All types of pathogen have a simple life cycle. They infect a host, reproduce themselves - or replicate themselves if they are a virus - spread from their hostThe organism lived on or in by a parasite. and infect other organismLiving entity, eg animals, plants or microorganisms.. They all have structural adaptationA feature of an organism's body which helps it to survive. that make them successful at completing their life cycles, enabling them to cause further disease.
Bacteria, fungi and protists can reproduce wherever the conditions are warm, with plenty of moisture, nutrients, and space. Viruses can only reproduce by invading living cells.
| Virus | Bacteria | Protist | Fungus | |
| Prokaryote or eukaryote | Neither | Prokaryote | Eukaryote | Eukaryote |
| DNA location | In a protein shell | Cell cytoplasm | Nucleus | Nucleus |
| Single-celled or multicellular | Neither (non-cellular). Can only reproduce inside another living cell. | Single-celled | Single-celled or multicellular | Mostly multicellular but some are single-celled |
| Prokaryote or eukaryote | |
|---|---|
| Virus | Neither |
| Bacteria | Prokaryote |
| Protist | Eukaryote |
| Fungus | Eukaryote |
| DNA location | |
|---|---|
| Virus | In a protein shell |
| Bacteria | Cell cytoplasm |
| Protist | Nucleus |
| Fungus | Nucleus |
| Single-celled or multicellular | |
|---|---|
| Virus | Neither (non-cellular). Can only reproduce inside another living cell. |
| Bacteria | Single-celled |
| Protist | Single-celled or multicellular |
| Fungus | Mostly multicellular but some are single-celled |
Diseases caused by pathogens are called communicable diseaseA disease that is caused by a pathogen and so is transmitted rather than inherited or caused by an environmental factor.. This means they can be transferred from one organism to another.
There are other types of disease that cannot be caught:
- inherited genetic disorders like cystic fibrosisA disorder that mainly affects the lungs, pancreas, liver, and intestine. The main symptom is difficulty breathing.
- deficiency diseaseA disease that develops because an organism (plant or animal) does not have enough vitamins or mineral ions. that are caused by a lack of essential vitamins or mineralsNaturally occurring, inorganic chemical substances. Minerals are necessary for both plant and animal health., eg scurvyA deficiency disease that occurs when an individual has insufficient vitamin C., which occurs when an individual has insufficient vitamin C
- diseases like cancer that develop as a result of exposure to carcinogenA chemical or other agent that causes cancer. or develop naturally as cell division occurs incorrectly
Disease transmission
transmission The spreading of pathogenic disease, for example by touch, food, water. of pathogens can occur in a number of important ways.
| Type of transmission | Examples |
| Direct contact | This can be sexual contact during intercourse or non-sexual contact, eg shaking hands. |
| Water | Dirty water can transmit many diseases, such as the cholera bacterium. |
| Air | When a person who is infected by the common cold sneezes, they spray thousands of tiny droplets containing virus particles, which can infect others. |
| Unhygienic food preparation | Undercooked or reheated food can cause bacterial diseases like Salmonella food poisoning. |
| Vector | Malaria is spread by mosquitoes. Any organism that can spread a disease is called a vector. |
| Type of transmission | Direct contact |
|---|---|
| Examples | This can be sexual contact during intercourse or non-sexual contact, eg shaking hands. |
| Type of transmission | Water |
|---|---|
| Examples | Dirty water can transmit many diseases, such as the cholera bacterium. |
| Type of transmission | Air |
|---|---|
| Examples | When a person who is infected by the common cold sneezes, they spray thousands of tiny droplets containing virus particles, which can infect others. |
| Type of transmission | Unhygienic food preparation |
|---|---|
| Examples | Undercooked or reheated food can cause bacterial diseases like Salmonella food poisoning. |
| Type of transmission | Vector |
|---|---|
| Examples | Malaria is spread by mosquitoes. Any organism that can spread a disease is called a vector. |
Causing symptoms
Pathogens cause symptoms by:
- releasing toxinA type of natural poison produced by an organism, often as a form of protection.
- damaging cells by invading and reproducing
It takes a while for pathogens to cause symptoms after they have infected an organism. This is because the symptoms only appear when there is a very large number of pathogens present. The time taken between infection and the appearance of symptoms is called the incubation period.
The incubation period can last for a few hours or even several years, depending on the pathogen. As they are not showing symptoms, the host organism can appear healthy.