A pathogen is an micro-organism that causes disease, the main forms of pathogen is bacteria and viruses.
Bacteria are one of the five kingdoms. Bacteria are really small - they are only 10 micrometres long - classed as microscopic organisms because of their size and these are the largest! They come in many different sizes. Bacteria have only one cell, which does not have a nucleus, the DNA is in the cytoplasm. They are living cells and with the correct conditions can multiply quickly. Once inside the body they will release toxins which is the reason you may feel ill.
Viruses are really, really small, and much smaller than bacteria. All they are is a fragment of genetic material inside a protein coat - which protects them. In the body, they reproduce inside host cells (they only can reproduce in another cell), forming thousands of other exact replicas of the virus, this leads them to fill up the host cell until it bursts and the virus now has access to the bloodstream and airways. Diseases caused by viruses include:
- influenza (flu)
- chicken pox
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