When I was a young kid, I used to annoy my dad with incessant questions of Why? “Why is the sky blue?” He couldn’t give me a satisfactory answer, and I had to go to college to find the answer. Now it’s payback time. My granddaughter is bombarding me with daily questions, like “why do men wear pants and women wear skirts?” and other such questions for which there are no good straightforward answers.

Here is a question I kept asking myself (in preparation for Rebecca’s inevitable question) and for which I didn’t have a satisfactory answer: Why do the flu epidemics occur only in the winter? To my delight, I discovered at least a partial answer in the March 2 of the online edition of Nature Chemical Biology.


Will a leopard change its spots?

When it comes to viruses, the answer is a definite maybe. A team of NIH scientists used a wonderfully named technique called magic angle spinning nuclear magnetic resonance to create detailed images of how the virus’s outer membrane responded to variations in temperature. The virus’s outer membrane is composed chiefly of lipids. As we discovered from deep observations in the kitchen, butter is solid in the refrigerator, gets softer at room temperature, and turns liquid in the frying pan. Dr. Zimmerberg and his colleagues found that at temperatures slightly above freezing, the virus’s lipid covering solidified into a gel. When temperatures reached about 60°F, the outer membrane began to thaw, and finally turned to a soupy mix.

Cooler temperatures, apparently, cause the virus to form the rubbery outer covering that can withstand travel from person to person. Once in the respiratory tract, the warm temperature in the body causes the covering to melt to its liquid form so that the virus can infect the cells of its new host.

In the winter, the lipid covering protects the virus from destructive environmental conditions when it travels in the fomites (small droplets formed when we sneeze or cough). In spring and summer, however, the temperatures are too high to allow the viral membrane to enter its gel state. The individual flu viruses apparently dry out and weaken, and the flu season wanes.


What are the implications?

I remember my old folks warning me not to stay in a hot room and then go out into the cold: You’ll get the “grippe” (French and Yiddish for the flu) was their anguished cry as I ran out to play on the street. Well, they were only partly right. Yes, when temperatures in the winter dropped to the 50’s and 40’s, I was much more prone to getting the flu (accompanied by the inevitable reproach of “I told you so”). But the heated room? This is exactly what is needed for protection against transmission of the flu virus. As the authors of the study theorized, in areas affected by a severe form of the flu, people might better protect themselves against getting sick by remaining indoors at warmer temperatures than usual. Let me add my two cents’ worth: Get a flu shot!


My take

The sophisticated study, for the first time, offers a plausible explanation for the winter flu epidemics. But still…

How do you explain flu epidemics in Florida? Can we blame it on the “Canadian geese” migrating from the Northeast?

Many other viruses contain lipids in their envelope, yet their spread is not restricted to winter.

There must be other factors that account for the puzzling phenomenon of winter epidemics, but this study is an important step forward in solving the puzzle.

Yes, Rebecca, I have the answer, in case you wondered.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.


  1. Historical Facts Believed To Be Associated With Influenza:The last influenza pandemic that occurred in the United States was nearly 100 years ago, and this deadly outbreak resulted in about 50 million deaths worldwide. The pandemic that occurred before this one happened about thirty years before the 1918 flu. Influenza epidemics typically occur about every 8 months or so. Influenza is caused by a virus, which is a parasite that needs a host to survive and reproduce. It was called the Spanish Flu because the first human case was identified there. The pandemic ended up killing more than those that died during WWI. Understandably there was panic among people worldwide, as influenza was not discovered until 1933, so the mystery was rather frightening of what was happening. Those who survived have allowed others to obtain antibodies from them to develop other antibodies for future viral outbreaks that may occur with this type of virus. This last influenza pandemic also allowed others to obtain this virus from those who died as a result to facilitate effective treatments and vaccines for viral outbreaks that may happen in the future as well. The virus responsible for the 1918 pandemic was an avian influenza. Nearly 700,000 people in the U.S. died as the result of the Spanish Flu- and those that did die was due often to a bacterial pneumonia that followed the viral invasion and damage. Ultimately, this pandemic killed nearly 3 percent of humans infected. Normally, an influenza strain may kill less than one percent of those infected. The Spanish Flu caused an unusually severe immune response in the human host which made it very deadly due to overkill of the cells of this host. The influenza viruses are categorized as A, B, and C. The Influenza A virus is the one that historically has caused pandemics that have developed-, such as the Spanish Flu Pandemic. The other influenza pandemics primarily have occurred in countries in Asia.With influenza, it is understood that the disease influenza is a disease caused by a RNA virus that can infect both mammals and birds. In fact, this particular virus can mutate to where it can be shared between the two life forms and multiply within each one of them. Unlike coryza, influenza expresses symptoms more severely, and usually lasts two weeks until one recovers who has the flu. Influenza, however, poses a danger to some with compromised immune systems, such as the chronically ill, so the recommendation is greater in such populations, along with women who may be pregnant during the flu season, residents of nursing homes or chronic care facilities. Health care personnel are encouraged to get the flu vaccine as well. Such populations allow influenza to progress to deadly pneumonia. Symptoms of influenza usually start to express themselves symptomatically about two days or so after being infected with the virus. Over 10 percent of the population is infected with this virus every year- resulting in about 200,000 hospitalizations and nearly 40,000 deaths. This season’s first influenza case was identified in Delaware in November of 2008, and it was a type B influenza strain.The flu vaccination is trivalent- meaning it contains three viral strains of suspected viruses for flu outbreaks during a particular winter season, as determined by the World Health Organization, as well as the Centers for Disease Control, and other organizations. Unfortunately, the influenza vaccine administered last flu season was largely ineffective due to unsuspected strains of the virus infecting others, although about 140 million injections of this vaccine were administered. After giving the vaccination dose to one, it takes about 10 days for that person to build up the immunity for the disease of influenza. The months of October to December are recommended to receive this vaccine. And the vaccine is about 50 percent effective in offering protection from influenza, according to others. Vaccines are a catalyst for antibody production in humans, which protect them against the virus. Influenza vaccines can be given by injection or nasally. Anti-virals, on the other hand, decrease greatly the ability for viruses to reproduce once established in a human.The Avian influenza that many have heard of is potentially the next flu pandemic- as humans have no immunity to what is called the H5N1 virus- on of about 1 strains of avian Influenza. For an Influenza pandemic to occur, which means a global disease existence, the virus must emerge from another species to humans without immunity, as well as the ability to make more humans ill than normal. Also, the virus must be highly contagious for a pandemic to occur. The H5N1 virus appears to replicate in the human GI tract and also has a longer incubation period in humans, one to two weeks, compared with other influenza strains. The H5N1 Avian influenza virus seems to have become progressively more pathogenic in the past decade, according to others.With the Avian Influenza existing with the H5N1 strain, millions of birds have been slaughtered due to the danger and unpredictability of this strain. The first human case infected with this strain occurred in China in 1997. The first human avian flu case outside of China was identified in 2003 in the Netherlands. The first recorded incidence of human-to-human transmission of the H5N1 virus was in Thailand in 2004. In 2006, it was discovered that the H5N1 had split into two separate strains. There have been outbreaks of Avian flu in about 15 countries in the world so far- with Indonesia being the worst. Migratory birds spread this influenza virus between continents. The pathogenic strength of the H5N1 strain varies due to constant re-assortment or switching of genetic material between the viruses- essentially creating a hybrid of what it was before this occurs. So far, about 300 people worldwide have been infected with this strain- and about half have died from the infection. Vaccinations are being developed and reformulated constantly at this time due to the pandemic threat of the H5N1 Influenza virus.Yet, the normal flu season that is now occurring was supplied with 150 million vaccines in the United States. However, some studies have shown that this vaccine is rather ineffective based on incidences of the acquisition of the influenza virus by others anyway.The influenza season peaks between the months of January and March. The vaccine for this influenza season is manufactured by 6 different companies. Yet the strains chosen are speculated influenza viruses, as this does not eliminate the chance of a new and dominant influenza viral strain that possibly could cause a pandemic. It takes manufacturers about 6 months to make and formulate the influenza vaccination. There is a vaccine for this illness that is produced every year according to which type of virus may be prevalent during a particular flu season. If influenza occurs in a human host, the results may be the patient acquiring pneumonia or meningitis.The presence of influenza can be widespread in certain states, yet not others. The vaccination is recommended to be administered to those who are at high risk, such as the chronically ill. Also, it is recommended that those under 18 years of age get the vaccine, as well as those people over the age of 50. Furthermore, those people who regularly take aspirin should receive the vaccine, as the influenza disease can become a catalyst for what is called Reye’s Syndrome. Pregnant women should receive the vaccine as well- as there are many other vaccines available to fortunately prevent other diseases, perhaps.http://www.cdc.gov/flu/weekly/Dan Abshear

  2. At present, without proof, shouldn’t our collective instincts suggest a virus has the cunning ability to jump into a warm body upon the detection of any cold. Once warmer weather returns the virus then vacates it’s winter residence and returns to wherever it would prefer to be, the trees, soil, refuse, etc. As far as the tropics go, maybe the reverse reaction of high heat drives the virus to seek out this same protection, as the body temperature in general remains constant.Experiments involving the cooling of bodies may prove this theory.(Alan Michael Elliott – Windsor, Ontario, Canada)

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