The Swine Flu (H1N1) Pandemic

Q: What is a pandemic?

An epidemic is an illness that spreads to many people, and a pandemic is an epidemic that spreads around the world.  When the words “epidemic” and “pandemic” are used, they often refer to a spreading infection caused by viruses or bacteria.  However pandemics can refer to non-infectious illnesses, such as the obesity epidemic caused by unhealthy lifestyle.

Pandemics typically are caused by a new type of infectious agent to which most people have not yet developed immunity. Pandemics often cause more severe disease than epidemics—but not always.

Q: What is “swine flu?"

A: Flu is a disease caused by the influenza virus. Humans, pigs, birds, and other animals all can be infected by influenza viruses. Typically, influenza viruses can infect only one species, so the influenza viruses of humans are different from those of pigs and birds. However, sometimes a virus can infect more than one species. For example, pigs sometimes can be infected not only with pig influenza viruses, but also with human and bird influenza viruses. Then these viruses can swap genes, creating new viruses that have a mix of genes—from human, pig, and bird viruses. That is what has happened with this new swine flu virus, which contains some genes from human, swine, and bird influenza viruses.

Sometimes this swapping of genes changes a virus from one able to infect only pigs or only birds to one that also can infect humans. When that happens, we refer to the illness as “swine flu” or “bird flu.” When a swine (or bird) influenza virus develops the ability to infect a human, it usually is not easily passed from human to human, and so an epidemic caused by the virus does not develop.

Early in 2009, a new swine flu virus emerged in Mexico that not only developed the ability to infect humans, but also to spread easily from human to human. It apparently is transmitted by sneezing and coughing and by skin-to-skin contact (like shaking hands or kissing) with an infected person.  It now has spread to nearly 200 countries around the world. On June 11, the World Health Organization stated that a global pandemic had developed

Q: Are swine flu or bird flu viruses dangerous?

A: When swine flu or bird flu viruses develop the ability to spread from human to human, they can be very dangerous: they can cause a pandemic, and they can produce severe disease.

One reason that pandemic illness often is more severe than the regular winter flu is that the virus is so new. The regular winter flu viruses that circulate each year are human influenza viruses and are similar to the viruses that have caused the flu in years past. As a result, most people have some degree of immunity to the latest regular human flu virus. The unusual swine flu or bird flu viruses that develop the ability for person-to-person spread are so different from regular human flu viruses that many people have little or no immunity to them. That is what experts think has happened with the new swine flu H1N1 virus.