What you need to know about the flu (including H1N1 or “swine flu”)

Brandel France de Bravo, MPH, Susan Dudley, PhD, Cecily Dvorak, MD, Jennifer Yttri, PHD, and Jessica Cote

Updated 2013

News about the flu is everywhere. First it was “avian flu” or “bird flu” in 2003, and in 2009 there was an outbreak of the H1N1 virus, originally called “swine flu.” In January 2013, the U.S. Centers for Disease Control and Prevention (CDC) declared the new 2012-2013  flu season an epidemic, which means it had caused over 7.2% of all deaths in the U.S. during the week before their decision.

Human flu—or influenza—is caused by a virus, usually striking in temperate climates like the United States in fall and winter. According to the CDC, influenza infects up to 20% of the population each year.  Approximately 200,000 people in the U.S. are hospitalized with seasonal flu every year, and tens of thousands may die from it—usually from infections like pneumonia that can be triggered by the flu.

Where does the flu come from, and how can we protect ourselves?

Influenza A viruses are the most common type of flu virus and the one responsible for seasonal flu and this century’s pandemics. Influenza A viruses can infect birds, humans, and other mammals. Some birds (wild ducks, for example) can serve as “hosts” for the virus. This means that they can spread it to other birds or farm animals without getting sick themselves. These viruses, which don’t infect humans, rarely pose a health. But if they mutate into forms that can also infect people, a severe flu outbreak can happen.

The main flu strain circulating this season (2012-2013) is H3N2. All influenza A viruses are given names made up of two letters and numbers. The letters—“H” and “N” —refer to the two main proteins on the surface of the flu virus. The numbers following the “H” and “N” refer to specific versions of each protein. With all the versions of “H” and “N” that scientists have identified, there could be 144 different combinations! Each year, a different combination of “H” and “N” can be found on the strain of flu virus circulating through the population. Not only does the flu change from one year to the next, but each year there are usually a few different strains of flu circulating at the same time.

The specific strains of virus that cause the seasonal flu are different every year, which is why we need to get annual flu shots instead of a one-time vaccination. Antibiotics don’t work on the flu, because antibiotics kill bacteria, not viruses. Public health experts keep close watch on the flu viruses from previous years and ones moving through animal populations to predict which ones are likely to spread to people. From that information, new vaccines are developed each year that will work against the particular viral strain that is expected. Vaccine production takes at least six months.

The Seasonal Flu Vaccine

This year’s flu vaccine, designed by scientists to fight the specific strains they expected to see this year, has been moderately successful: it is 62% effective, according to the CDC, which means that 38% of people who receive the shot still get sick with the flu. Some people who get the flu shot can still get sick because, although each flu shot is equipped to prevent many current flu strains, some strains change quickly over time, and their new shape is different enough to dodge the body’s anti-virus “roadblock” put in place by the flu shot.  Also, sometimes scientists don’t notice one particular strain when they are designing the vaccine and so it isn’t included in the flu shot.

Worried about getting the flu? Pharmacies all out of the flu vaccine? Here are recommendations from the CDC on how to stay healthy during flu season:

  1. Avoid close contact. Avoid close contact with people who are sick. When you are sick, keep your distance from others to protect them from getting sick too.
  2. Stay home when you are sick. If possible, stay home from work, school, and errands when you are sick. You will help prevent others from catching your illness.
  3. Cover your mouth and nose. Cover your mouth and nose with a tissue when coughing or sneezing. It may prevent those around you from getting sick.
  4. Clean your hands. Washing your hands often will help protect you from germs. If soap and water are not available, use an alcohol-based hand rub.
  5. Avoid touching your eyes, nose or mouth. Germs are often spread when a person touches something that is contaminated with germs and then touches his or her eyes, nose, or mouth.
  6. Practice other good health habits. Clean and disinfect frequently touched surfaces at home, work or school, especially when someone is ill. Get plenty of sleep, be physically active, manage your stress, drink plenty of fluids, and eat nutritious food.

And if you think you are getting sick with the flu, follow these tips from the CDC:

  1. Drink lots of clear liquids like water to prevent dehydration.
  2. Treat a fever with cool washcloth compresses, a lukewarm bath, or over-the-counter fever reducers.
  3. Treat a cough with a humidifier or over-the-counter cough medicines.
  4. If you are vomiting or have diarrhea, eat plain foods like crackers and drink lots of water.

If your symptoms worsen and become very severe, call your doctor.

Seasonal, Pandemic, and Rare Flu Cases: What’s the difference between them?

Widespread flu during the fall and winter months is not new or surprising. Every year flu season happens between October and May in the U.S.

When flu spreads, infecting an exceptionally high number of people all over the world, it is called a pandemic. Pandemic flu is usually caused by one strain of flu virus. Flu pandemics have occurred ten times in the last 300 years, with some of the most famous occurring in 1918-19, 1957-58 and 1968-69. Pandemic flu can be frightening. The World Health Organization declared the H1N1 flu a pandemic in 2009. H1N1 is not the usual type of seasonal flu. In 2009, existing flu vaccines provided no protection against it. 2009 H1N1 is a brand new flu that combines genes from four types of flu: two strains of flu that normally only affect pigs, mixed together with genes from a flu that strikes birds and a flu that is typical among humans. At first, H1N1 was called swine flu because genes from two swine flu viruses are present, but the H1N1 is spread from human to human and you can’t get it from eating pork or bacon.

Pandemic flu passes from person to person just like seasonal flu. The percentage of flu-infected people who die from it can be comparable to seasonal flu. The worry is that the virus may change and become stronger since more people get sick from it.

For the past 20 years, influenza A viruses from two main “families” have predominated as the cause of annual human flu outbreaks. Sometimes rare flu viruses can be found, like the avian flu or H5N1. What makes a rare flu virus different from the seasonal flu that we are already familiar with is that it comes from a virus family that has not been known to infect humans before. This means that no one has developed any natural immunity based on previous exposures. In addition, this form of the virus can cause more severe responses in the human body than we normally expect from the flu. This is why H5N1 virus was so deadly.

Pandemic H1N1 is similar to the rare bird flu in some ways but not in others. While avian flu (H5N1) has a very high fatality rate—over 60% of people infected with it died—very few people caught avian flu from other people. Most of the people who have become sick from avian flu contracted the virus directly from birds-usually because they are involved in raising or selling birds for food.

The good news about H1N1 is that it has not yet proved to be anywhere near as deadly as bird flu. The bad news is that it spreads from person to person just like any other flu, and over time it may become more harmful. People who study viruses are concerned about H1N1 and H5N1 because unlike the seasonal flu, these two viruses are very unstable, which means they are always looking to “borrow” genetic material from elsewhere. By incorporating new genetic material, they can potentially grow stronger and harder to control with vaccines and anti-viral medications like Tamiflu (oseltamivir) or Relenza (zanamivir).

Both H1N1 and H5N1 caused panic because of certain similarities they have to the strain responsible for the 1918 flu, which killed more than 50 million people worldwide. When bird flu first began to spread in 2003, it was thought that the 1918 epidemic was caused by a virus strain that jumped from birds to humans directly. Based on new evidence, experts now believe that the virus traveled to humans from birds via pigs. By passing through pigs first, the virus was able to better adapt itself to humans.

Even though pandemic and rare flu may be different from seasonal flu, you can reduce your risk of catching and spreading all flu by practicing the same precautions: by washing your hands regularly, covering your nose and mouth when you cough or sneeze, staying home and away from others when you are sick, and by getting the vaccine once it’s available if you fall into any of the targeted groups listed in the chart below. With any type of flu, the people who suffer most and can die from it are infants, the elderly, and individuals with chronic illnesses such as asthma, heart disease, diabetes, obesity, or compromised immune systems (due to an immune disease like HIV/AIDS, or due to taking medications that suppress the immune system).

Here is a side-by-side comparison of the seasonal flu, a pandemic flu (2009 H1N1), and a rare flu (avian flu H5N1).



(Ex.: 2009 H1N1)


(Ex.: Avian flu H5N1)

How it spreads 
Droplets from an infected person’s nose or mouth get into a healthy person’s nose, mouth, or eyes.
How it spreads
Droplets from an infected person’s nose or mouth get into a healthy person’s nose, mouth, or eyes.The 2009 H1N1 swine flu was the first pandemic flu in 40 years. 2009 H1N1 combined avian and swine flu with human flu to make a new virus that had not been seen before.
How it spreads
Flu viruses are normally found in animals, like pigs or birds, and sometimes the animal versions can spread to humans. Most people get rare flu viruses through direct exposure to infected animals. The virus poses little danger to humans unless it develops the capacity to spread directly from person to person.
Flu symptoms are often similar to, but more severe than, a “common cold.” They include coughing, sore throat, and fever. Most people recover in 1-2 weeks, but some develop pneumonia or other very serious complications requiring hospitalization.
They appear similar to the seasonal flu: cough, sore throat, congestion or runny nose, headaches, body aches, chills and fever. Some people vomit or have diarrhea.Pandemic flu infects a larger number of people than seasonal flu and can result in higher death rates. As of March 2010, there had been 12,000 deaths from H1N1 in the U.S.
Because this virus has not yet mutated to a form that can easily spread in human populations, it is not clear what symptoms it will cause if that happens. Scientists predict that it could be quite severe, producing high rates of life-threatening complications, especially affecting the lungs.
The bad news
With annual variations in types of flu, tens of thousands of Americans die and more than 200,000 are hospitalized for flu-related complications every year. Most Americans don’t get flu shots.
The bad news
The World Health Organization declared 2009 H1N1 a pandemic. The virus was much less active in the U.S. during the summer of 2009 compared to in South America where it was winter.  The flu spread in the Northern hemisphere in the fall and winter, possibly in a more virulent form. This flu, like the much rarer avian flu, was more likely than seasonal flu to affect the lower respiratory tract, going deeper into the lungs. This sometimes resulted in an aggressive pneumonia. H1N1 infected a larger number of people than seasonal flu.
The bad news
Approximately 60% of the people infected with the H5N1 virus died. In contrast, the “Spanish flu” epidemic of 1918, from which an estimated 50 million people died worldwide, had a much lower death rate.
The good news
Because most annual flu viruses are related to previous viruses, people tend to build “partial immunities” to the viruses that spread each year. That means their immune systems will be able to figure out how to resist new flu infections more quickly than if it were something drastically different from what they’ve been exposed to before.
The good news
H1N1 was not more dangerous than seasonal flu. A vaccine was ready by the fall of 2009. As for any pandemic flu, the populations that will be given priority for vaccination are: health care workers; pregnant women; caregivers for children under 6 months of age (children this young cannot receive the vaccine); children and young adults from 6 months to 24 years; and people 25 to 64 with underlying health problems, such as asthma, diabetes or heart disease.
The good news
Scientific and public health communities all around the world are on alert to pick up the first signs if new, rare flu viruses, like the H5N1 virus, become a serious threat to humans. Efforts are in place to develop new anti-viral medications and vaccines in case it begins spreading among humans.
What you should do

  • Get a flu shot every year.
  • Wash your hands often.
  • Cover your mouth and nose when you sneeze or cough.
  • Keep your immune system strong through exercise, healthy food, and plenty of sleep.
  • Although you can be contagious with the flu before your symptoms develop, stay home when you’re sick to avoid spreading it to even more people.
What you should do

  • Make sure family members who fall into the above categories get vaccinated.
  • Wash your hands often.
  • Cover your mouth and nose when you sneeze or cough.
  • Keep your immune system strong through exercise, healthy food, and plenty of sleep.
  • Keep sneezing, coughing and ill family members at home and away from others.
  • You can buy a surgical mask or respirator for sick family members but their effectiveness in reducing the spread of flu has not been sufficiently studied.
What you should do

  • Your family is NOT in imminent danger from this strain of flu.
  • Follow the news and stay alert for public health advisories.
  • Continue to follow all recommendations for avoiding seasonal flu.

Symptoms to watch out for

Most people who get the flu, regardless of the kind, feel miserable for some days and have to stay home from work or school. Usually the best treatment is to get bed rest, drink lots of fluids, and if you have a fever or a headache, take aspirin, acetaminophen, or ibuprofen (aspirin should not be taken by children and teens with flu-like symptoms). For individuals with underlying medical conditions, very young children, and older people, the flu can result in serious illness and be life-threatening. If a family member fits any of these categories and begins to have flu-like symptoms, you may need to visit a doctor, medical center or hospital. There is no way to know whether the flu your loved one has is the seasonal flu, H1N1, or a rare flu virus without a laboratory test. He or she may be prescribed anti-viral medication such as Tamiflu or Relenza, or be kept under a doctor’s close supervision. These drugs are reserved for people who are particularly vulnerable to, or sick with, the flu and cannot be bought over the counter.

In children with the flu, watch out for dehydration (if the child refuses to drink); blue/gray lips, skin or fingernails; rapid breathing; extreme sleepiness, irritability or confusion. These symptoms mean the child needs immediate medical attention. Some flu viruses, like H1N1, can cause more severe symptoms such as seizures in children.

The symptoms to watch out for in adults include chest pain, shortness of breath, sudden dizziness, and confusion. Severe or persistent vomiting also can be serious-in children and adults. And anytime a person with the flu seems to be recovering and then takes a turn for the worse, developing a fever (or a recurrence of fever) or a more pronounced and painful cough, he or she should get medical attention because another infection may be present.

This article will be updated regularly.