by Alison Bowler

Created on: April 01, 2009   Last Updated: August 18, 2009

Some viruses, such as smallpox, can infect only humans. While others, such as rabies, are primarily animal pathogens but can infect other animals or humans. Other viruses, such as myxomatosis in rabbits, will only infect specific animals. When a pathogen that normally infects one animal infects another it is said to have "jumped the species barrier".

There are three types of Influenza virus that cause infection in humans. Two of these viruses, Influenza B and Influenza C, have only been found causing infections in humans and not any other animals. The third virus Influenza A, however, is well known for infecting birds as can be seen by the current concern with the spread of bird, or as it should be called avian, "flu" among domestic and wild fowl. There are in fact many more strains of avian Influenza virus than there are strains of human Influenza.

When a strain of the Influenza virus "jumps the species barrier", it can be deadly. Persons in close contact with infected domestic fowl have sometimes developed a fatal pneumonia when they contract an avian "flu" strain. Fortunately, at present, human-to-human infections of this deadly avian strain are rare. The massive Influenza pandemic at the end of World War One in 1918, although known as the "Spanish Flu", is thought to have originally started as an avian virus, which infected man. This strain was so aggressive that more people died in India alone from the infection than died worldwide as a result of fours years of war. People infected with the Spanish Flu frequently died of pneumonia within twenty-four hours of developing the first "flu" symptoms.

Pigs can also be infected with some strains of the Influenza A virus. It is also possible for humans to catch the "flu" from pigs. This has led to the designation of "Swine" Flu being given to the viral influenza strains spreading through the human population in 1977 and 2009. Horses can also be infected with certain strains of Influenza A.

Not all strains of Influenza A can infect humans. The ability of the virus to infect the cells of a potential host depends on the shape of receptors on the walls of the host's cells. These receptors bind to the neuraminidase and hemagglutinin protein spikes on the outer surface of the virus. If the cell wall receptors are not the correct shape then the virus will fail to bind and cannot infect the cell. This requirement of the Influenza virus prevents it from infecting many other animal species.

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