Viruses, small biological particles that have the ability to reproduce, have been around far longer than even humans, existing for billions of years. Unlike living organisms, viruses cannot reproduce independently; rather, they depend on the infection of a host organism. As humans evolve, the viruses evolve as well, and new strains continually arise over time. The Human Immunodeficiency Virus (HIV, the causative agent of autoimmune deficiency syndrome, AIDS) and the Severe Acute Respiratory Syndrome (SARS) virus are two examples of viruses that have only recently spread to humans. The newest virus making headlines is the H1N1 flu, which has fast become a topic of international concern.

What is the flu?

Influenza, or the flu, is a disease caused by the influenza virus. Although influenza is often confused with other influenza-like illnesses, especially the common cold, influenza is a more severe disease, and is caused by a different virus (rhinoviruses cause the common cold). The most common symptoms of the disease are chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort.

Influenza is highly contagious and is easily spread from person to person. It is transmitted by droplets from the throat or nose of an infected person, spread typically by sneezing or coughing. These droplets can settle on mucuosal surfaces of healthy people by either inhalation – to people within 6 feet of the infected individual, according to the Occupational Safety and Health Administration – or indirect contact. An example of this would be touching surfaces contaminated with influenza virus and then touching the eyes, nose or mouth. The time that the infected individual may be contagious varies, depending on the health of the person infected and the strain of influenza. Typically, healthy adults may be contagious from 1 day before getting symptoms to approximately 5 days after symptoms begin.

The seasonal flu is not typically fatal in healthy individuals. It normally only poses a threat for the elderly and the very young, two populations that have a suppressed immune system. Individuals with AIDS or other autoimmune diseases are also at risk, as they are also less-able to fight off an influenza infection due to a damaged immune system.

What Makes the Swine Flu Unique?

The danger of the influenza virus lies in its unique ability to rapidly adapt and change. There are variants of the influenza virus that infect animals, such as chickens (and related birds) and pigs. Normally, the virus spreads only to other members of the same species, and thus does not pose a serious threat to humans. However, in a small number of cases, an animal can become infected with two different types of influenza virus – say, one from a human and one from a pig. When this occurs, the viruses can mix together, ultimately forming a new type of virus in a process called re-assortment. The virus produced contains some characteristics of both parental viruses (much in the way human children contain characteristics of both parents). The properties of such a virus are impossible to predict – sometimes the virus may be harmless, and other times, it may have catastrophic effects.

When a single influenza virus infects a cell, it sheds its coat of outer proteins and its RNA genome (the functional equivalent of DNA) moves into the cellular nucleus, which houses the organism’s DNA. It is possible for not only a single virus, but multiple viruses to enter a single cell – viruses of either the same or different types. When the viral RNA enters the cell’s nucleus, it makes many copies of itself, which become packaged into new viral particles. These particles then exit the infected cell to ultimately infect other cells. During the packaging process, the virus has no way to choose the RNA that is included in the viral particle – each piece of viral RNA that escapes from the cell nucleus looks essentially the same to the virus. Therefore, if two or more different types of viruses infect the same cell, the viral particles created from this cell will contain all sorts of combinations of RNA from the two “parental” viruses. In many cases, such hybrid viruses will not be able to function, since its component parts will not be able to work properly together. The process can be likened to entering a kitchen full of ingredients and mixing them together at random: while most combinations would be a mess, a few combinations may be innovative and more effective than the desired recipe. In the same fashion, some combinations create functional viruses which are entirely new to nature and whose effects are difficult to predict.

Swine flu – what is it?

The so-called swine flu (or H1N1 flu) is similar to the regular seasonal flu, with a few key differences. Since it began spreading internationally in April 2009, scientists have been working fervently to decipher the virus and its origins, but much remains to be understood.

It appears that the H1N1 flu is a mixture of human, avian, and swine influenzas, a combination that resulted from viral re-assortment. This means that the virus contains genetic information from an avian flu virus, a swine flu virus, and a human flu virus. However, because the flu was first observed in pigs, it was incorrectly dubbed “swine flu.” The H1N1 flu appears to spread the same way as the normal seasonal flu – through coughing and sneezing. Cases have been reported in all fifty US states and in many countries worldwide, with the number of confirmed cases and mortalities still increasing daily. The World Health Organization (WHO) is currently preparing to raise the pandemic alert to phase six, its highest level. While this may sound alarming, it is important to realize that this alert indicates only that there is widespread human infection around the globe, and is NOT a measure of the mortality or general harmfulness of the virus.

Can flu shots protect me? Why or why not?

Seasonal flu shots are recommended annually to at-risk populations, such as immunocompromised individuals and those with a higher risk of exposure to influenza. The ability of the influenza virus to change makes it impossible for a single vaccination to prevent all types of influenza. However, scientists can often predict which types of human influenza will be the most prevalent each year, and the vaccine is made to target these subtypes. However, the swine flu is only part human flu, so it is rather different than the types of flu that have been targeted by past vaccines. Therefore, currently available flu shots offer little to no protection against the swine flu. However, the development of an H1N1 flu vaccine is currently underway, and may be ready by as soon as this summer.

Fortunately, influenza is rarely fatal in healthy individuals. Both seasonal and H1N1 flu usually go away within 5-7 days, and antiviral treatments can help to moderate disease symptoms and to shorten the length of the illness.

So if it is not that harmful, why is it getting so much attention?

While it remains somewhat mysterious, the swine flu appears to be similar to other viruses that have caused pandemics in the past, such as the Spanish flu virus in 1918 or the 1957 Asian flu and Hong Kong Flu (1968-1969), two of the more recent flu epidemics, which killed millions of people worldwide. Both of these epidemics were caused by viruses that had undergone rearrangements, like the current H1N1 virus. The fact that the virus, which originated from animals other than humans, can now spread extensively from human to human, is worrisome because it creates the potential for the infection of large populations of individuals who have no protection. It is also unknown how the virus will respond – as the virus is propagated, it can continue to change and evolve, picking up new characteristics. Despite these fears, to date, concerns about the H1N1 virus have not come to fruition – while the H1N1 flu appears to be highly contagious, it currently is no more severe than the seasonal flu.

Will we see it again?

Though media coverage of the swine flu has waned in recent weeks, the swine flu has not disappeared. Much like the seasonal flu, which appears to come and go with the seasons, we likely have not seen the last of the swine flu. The flu is being spread from person to person throughout the entire year, but for unknown reasons, cases of influenza spike roughly ten-fold during the so-called flu season, which occurs in the winter. Just as the cases of the normal, seasonal flu will spike in the winter, swine flu cases likely will spike as well. Therefore, in the southern hemisphere, which has its winter from June to August, the number of swine flu cases is expected to rise continually through these months. The impacts of such increases are impossible to predict with certainty. However, there are global efforts designed to prevent the repeat of previous world pandemics, and scientists worldwide are working around the clock to better understand and handle any problems that may arise.

–Kevin Beier, Harvard Medical School

For More Information:

Centers for Disease Control and Prevention:
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Swine Flu to Spread in Southern Hemisphere, from
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Primary Literature:

Eccles, R. Understanding the symptoms of the common cold and influenza. Lancet Infect Dis 5(11), 718-25 (2005)

Taubenberger, J. and Morens, D. 1918 Influenza: the mother of all pandemics. Emerg Infect Dis 12(1), 15-22 (2006)

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