Candida albican on an agar plate

Old and new challenges

Which organisms can cause infections?

For many kinds of organism the human body is a perfect environment in which to live. We have learned to cope with the presence of most organisms that like to live inside us, but some are harmful or deadly. Among the most deadly are organisms that have only recently made us their home – we generally have little or no defence against them.

The bad news is that we can expect one or two new human infections to appear every year.


E.g. HIV, Ebola: There were 1.6 million AIDS-related deaths globally in 2012. By early November 2014 nearly 5,000 people were reported to have died in the Ebola epidemic that started in February 2014 in West Africa.


E.g. methicillin-resistant Staphylococcus aureus (MRSA), Neisseria gonorrhoeae: Antibiotic resistance is a growing threat worldwide. S. aureus is the leading cause of hospital-associated infections, with MRSA causing a high percentage of these. By 2014 ten countries had reported failures of last-resort treatments for gonorrhoea.


E.g. Toxoplasma gondii, Entamoeba: Infections can be caused by eating contaminated foods. T. gondii also infects cats and can be picked up from cat litter boxes.


E.g. Candida (above), Cryptococcus, Aspergillus: Often a problem in specific populations – such as Cryptococcus in HIV-positive people or Aspergillus in people being treated for cancer or who have lung diseases.

Prion protein

E.g. vCJD: Responsible for 177 deaths in the UK between 1995 and 2013.

Lead image:

Candida albicans on an agar plate.

mostly*harmless/Flickr CC BY NC


Further reading

About this resource

This resource was first published in ‘Epidemics’ in September 2007 and reviewed and updated in January 2015.

Microbiology, Health, infection and disease, Medicine, Immunology
Education levels:
16–19, Continuing professional development