The flu is a common illness in the world. In general, nobody likes getting the flu, but aside from the very young, very old, or immuno-compromised, few people in the developed world worry that the flu is going to kill them. In general, that calm feeling is well grounded, but occasionally, flu strains jump species from animals into humans, creating serious outbreaks of deadly swine- or bird-flu,
Zoonosis and Animals
Flu viruses demonstrate a capacity for zoonosis, essentially the leaping of viruses from an animal to a human host. Many viruses are extremely host specific and only work on certain animals, but random mutation and close contact with sick animals can always result in the emergence of new diseases.
New diseases have an advantage over the old diseases in that our immune systems may not be ideally suited to them. We haven’t had the thousands of years it takes to develop natural resistance, and lack of prior exposure means that the entire population is susceptible to catching the disease.
Some things that we do for modern convenience increase these risks. Keeping animals in close contact with each other, as they are in factory farms for chickens and pigs, allows viruses to spread rapidly and attack large populations. While this doesn’t guarantee the ability to zoonose, it does increase the probability of it. People working closely with the animals in overcrowded conditions makes this worse. Being in close contact with animals is generally not a good idea if you are afraid of zoonotic diseases.
Influenza and Evolution
The flu itself has abilities that improve its zoonotic chances. One of the primary drivers of the evolution is its uncanny ability to mix and match. When multiple strains of influenza are present in a host cell due to concurrent infection, the new viruses that are assembled within the host cell may be mixtures of the first two. This is effectively like creating a hybrid influenza within the cell.
Imagine a group of people with the flu are exposed to pigs with swine flu. There is a small but definite chance that either group could pass on their flu to the other. The new flu created within the pigs or people could be a zoonotic virus, and because it is partially swine in origin, it would be completely new to our immune systems. The new flu would be (potentially) extra viral, and spread extremely quickly against the unprepared population.
Historical Flu Pandemics
Flue pandemics have hit us before with devastating results. The Swine Flu of 1918 was so devastating that it may have been one of the major factors ending World War I. In that case, the swine flu, by a mechanism called a cytokine storm, killed not the immunodeficient but actually the healthiest of people. It killed at least 8 million in Spain alone, giving it the name “Spanish Flu.”
Recently, a far less deadly H1N1 strain became a problematic virus around the United States. While it killed far fewer people, it was still deadly in a number of cases, and it alerted us to the dangers that the simple flu can pose. Zoonotic diseases like the flu will always be with us, and we need to be sure to be prepared for a quick response in the event of an outbreak here or abroad.
Scitable.com: Genetics of the Influenza Virus
World Health Organization: Zoonoses and Veterinary Public Health.
Standford University: The Influenza Pandemic of 1918