Virtual Genetics Education Centre

Infection and diseases

Infection and diseases for schools and colleges

Bacteria have a series of different genes coding for characteristics that enable them to cause disease. These characteristics are often required in sequence for bacteria to successfully infect and cause damage to us.

Our bodies are home to millions of different bacteria. Many of these never cause any harm and live in our bodies quietly. Some of them can even help us, for example by aiding digestion of food in the gut. We refer to these bacteria as commensals. When we become infected by bacteria and contract disease, this is usually the result of these bacteria getting into the wrong parts of the body. For example, Nesseria meningitidis is one of the main causes of bacterial meningitis. This bacteria lives in the throats of roughly 10% of the healthy population, but only causes disease when it infects the membranes around the brain and spinal cord. This bacterium can also be dangerous if it gets into the blood, where it can cause sever septicaemia. We call bacteria that are capable of causing disease pathogens. Some bacteria that are usually commensals can go on to cause disease when our immune systems become compromised, following chemotherapy or HIV-1 infection for example. We refer to these as opportunistic pathogens.

Our bodies are home to millions of different bacteria, many of which never cause disease.

Causing disease relies on a sequence of events and many different features of bacteria are required for this process. The bacteria need to get to the right part of the bodies and begin to settle there (colonisation). They need to  invade the cells and tissues in that part of the body (invasion) as this is where they can cause damage. Bacteria need to multiple rapidly, to produce more disease causing cells (proliferation). They need to transmit to other parts of the body and to new healthy individuals (transmission).

Finally, throughout this time, the immune system is constantly trying to prevent pathogens from causing disease. As a result, pathogens have had to evolve different ways to try and escape the immune system. We refer to these as immune evasion.

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