Summary to Session 3
Now that you have completed Session 3 you should be aware of the incredible range and diversity of pathogens that infect humans and cause disease. This gives you some idea of the challenge faced by scientists and health care professionals in treating and preventing infectious diseases.
We have introduced you to the multicellular endoparasites; four categories of worm (roundworms and hookworms, tapeworms, filarial worms, and flatworms or flukes) that cause a variety of diseases by invading vital organs or living in the gut, bloodstream or tissues. Smaller in size are protists – unicellular pathogens that cause conditions such as sleeping sickness, leishmaniasis and malaria). Next you learned about fungal pathogens and bacteria. Pathogenic bacteria cause a wide range of diseases (e.g. cholera and tuberculosis) while ‘friendly’ commensal bacteria can be protective against pathogens. You should be aware that the antibiotics used to treat pathogenic bacteria also kill commensal bacteria, which can increase the risk of fungal and other infections. Finally, you have been introduced to the two smallest types of pathogen – viruses and prions – which are not formed from cells so are not considered to be alive. Diseases caused by infectious prions are relatively rare but the same cannot be said about viruses, which cause some of the most common infectious diseases such as influenza. Viruses also cause a considerable proportion of emerging infectious diseases and exhibit a remarkable range in their modes of transmission.
Taking all this together, it should be clear to you that there is no such thing as a generalised pathogen – even within the same category of pathogen they tend to behave differently! It is therefore unsurprising that the challenge to human health posed by infectious diseases shows little sign of abating.
You can now go to Session 4.