Breaking barriers

Some pathogens stick to their favourite hosts

If your pet cat gets the flu, fear not – your furry friend will not pass it on to you. The feline influenza virus does not infect human cells. And when foot and mouth disease hit the UK, there were no fears around human health – the virus almost never infects humans.

Barriers between species prevent many infectious diseases spreading from one species to another. A strong immune response might repel an attempted attack, or a pathogen may not be able to survive in another species.

Some pathogens are extremely versatile: certain bacteria, for example, can be found in soil as well as the human gut. Others are less so: not only are viruses often limited to one or a few species, they may also be limited to a narrow range of cell types within their hosts. The cold-sore virus, for example, can invade only nerve cells. Infectious agents usually recognise a cell of the ‘right type’ by specific receptors on the cell surface.

Illustration of HIV binding to the surface of a cell

HIV needs to bind to specific receptors to enter cells.


Illustration © Glen McBeth

Sometimes a pathogen will cross the species barrier, with potentially disastrous consequences. The Ebola virus has been detected in various species of bat, and the 2007 outbreak that killed 186 people in the Democratic Republic of the Congo was directly linked to fruit bats. There is also speculation that the 2014 outbreak started in bats: the family of the first victim, infected in Guinea, hunted bats for food. Gorillas and chimpanzees also harbour the virus but are not thought to act as reservoirs for the human disease.

Jumping the species barrier can occur in stages. With avian flu, for example, some strains can infect people but are not transferred from person to person (only from bird to person). This does not mean that they will not evolve to do so in the future.

Infections that have recently jumped species can be particularly deadly. Certain strains of avian flu, for example, are harmless to wild birds but deadly to captive birds such as chickens – and to humans. In part this may be because there has been no chance for hosts to evolve any form of resistance.

Scientists are carefully monitoring pathogens that may cross over in future. Farmers have been advised to take precautions when dealing with livestock infected with Schmallenberg virus, which causes stillbirths in cattle, sheep and goats. Related viruses can infect people but, as yet, there is no evidence that it poses a threat to human health.

Diseases that can be transmitted from animals to people are known as zoonoses. ‘Big Picture’ has lots of articles that discuss specific zoonoses, so why not try using our search box to learn more about one in particular, such as tuberculosis, Ebola or influenza?


About this resource

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

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