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What is a Pathogen?

A pathogen is an agent that causes infection or disease. Many diseases that affect animals and plants are caused by bacteria or viruses. However, pathogens can also include eukaryotic organisms, for example certain types of fungi and protoctists. An example of pathogenic fungi is Trichophyton mentagrophytes and Trichophyton rubrum – both are commonly known to cause athlete’s foot.

Like we have previously mentioned, not all bacteria are harmful to humans; however, some are. Pneumococcus is an example of a pathogenic bacterium that we have mentioned in the previous chapter. Pneumococcus is a bacterium that causes pneumonia.

A virus is a non-living organism. All viruses are parasitic and are therefore known as parasites as they can only reproduce inside living cells. Viruses can infect every type of living organism, including bacteria. The particular cell that the virus lives in is known as the host. Like many living organisms, there are many different types of viruses; however, we do not refer to these different types as ‘species’. This is because viruses are not cells and are not classed as living organisms as they do not carry out any of the ‘eight life processes’ other than that of reproduction, and even then viruses only reproduce parasitically. A virus is a particle that is very simple compared to an average cell; they have no cellular structure and instead have a protein coat that contains a fragment of genetic material – the nucleic acid (either DNA or RNA).  As well as being simple, virus particles are much smaller than bacteria and come in many different shapes and sizes. Take a look at the following diagram that illustrates a cell that has been infected by a virus:

inside living cells.

Remember, a virus only consists of a protein coat that contains a fragment of genetic material. In the diagram above you can see an envelope and spikes. These are not features of a virus themselves and are instead stolen from the host cell.

A virus particle reproduces by infecting a host cell and taking over its genetic machinery so that it can reproduce more virus particles. The host cell will eventually die when the virus has made enough virus particles so that they can be released to infect more living cells. The body’s immune system will normally destroy the virus before any long-lasting damage is caused; an example of this will be the influenza virus (‘flu’). However, with certain viruses that attack the immune system, a virus can cause serious damage or even be fatal; an example of a virus that attacks the human immune system is HIV (Human Immunodeficiency Virus), which can eventually cause the disease AIDS (Acquired Immune Deficiency Syndrome).

Remember that viruses do not just infect humans but also any other living organism; this includes other animals, plants, fungi, protoctists and even bacteria.

One example of a virus that infects a plant is the tobacco mosaic virus that prevents a plant’s ability to form chloroplasts. This leads to the discolouring of the leaves of tobacco plants.

tobacco plants.
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