Flu (Influenza) and H1N1 Symptoms & Causes

What causes influenza?

An influenza virus is generally passed from person-to-person by airborne transmission. This means your child can contract the flu by coming in contact with airborne viruses from an affected person by way of sneezing and coughing. The virus can also live for a short time on objects such as doorknobs, pens/pencils, keyboards, telephone receivers, and eating or drinking utensils, for example.

People are generally the most contagious with the flu 24 hours before they start having symptoms; that is why it is hard to prevent the spread of the flu, especially among children, because they do not always know they are sick. The risk of infecting others usually ends around the seventh day of the infection.

Different strains of flu

Influenza viruses are divided into three types designated as A, B, and C. 

  • Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates of hospitalization and death. Efforts to control the impact of influenza are focused on types A and B.
  • Influenza type C usually causes either a very mild respiratory illness or no symptoms at all. It does not cause epidemics and does not have the severe public health impact that influenza types A and B do.
  • Novel H1N1 (referred to as "swine flu" early on) is a new influenza virus causing illness in people. This new virus was first detected in people in the United States in April 2009. For the latest information on H1N1, please visit the Boston Public Health Commission, the Massachusetts Department of Public Health and the Centers for Disease Control.

Influenza viruses continually mutate or change, which enables the virus to evade the immune system of your child. People are susceptible to influenza infection throughout their lives. The process works as follows: 

  • A person infected with influenza virus develops antibodies against that virus.
  • The virus mutates or changes.
  • The "older" antibodies no longer recognizes the "newer" virus.
  • Reinfection occurs.

The older antibody can, however, provide partial protection against reinfection. Currently, three different seasonal influenza viruses circulate worldwide: two type A viruses and one type B. Immunizations given each year to protect against the flu contain the influenza virus strain from each type that is expected to cause the flu within that year.

Is the H1N1 flu more severe than seasonal flu?

With seasonal flu, we know that seasons vary in terms of timing, duration and severity.

  • Seasonal influenza can cause mild to severe illness, and at times can lead to death.
  • Each year, in the United States, on average 36,000 people die from flu-related complications and more than 200,000 people are hospitalized from flu-related causes.
  • Of those hospitalized, 20,000 are children younger than 5 years old.
  • Over 90% of deaths and about 60 percent of hospitalization occur in people older than 65.

When the novel H1N1 outbreak was first detected in mid-April 2009, CDC began working with states to collect information regarding the novel H1N1 flu outbreak.

  • Novel H1N1 flu has caused greater disease burden in people younger than 25 years of age than older people.
  • At this time, there are few cases and few deaths reported in people older than 64 years old, which is unusual when compared with seasonal flu.
  • Pregnancy and other previously recognized high risk medical conditions from seasonal influenza appear to be associated with increased risk of complications from this novel H1N1. These underlying conditions include asthma, diabetes, suppressed immune systems, heart disease, kidney disease, neurocognitive and neuromuscular disorders and pregnancy.

What are the symptoms of influenza?

Each child may experience symptoms differently, but common symptoms include:

How is a cold different from the flu?

A cold and the flu (influenza) are two different illnesses. A cold is relatively harmless and usually clears up by itself after a period of time, although sometimes it may lead to a secondary infection, such as an ear infection. However, the flu can lead to complications, such as pneumonia and even death. What may seem like a cold, could, in fact, be the flu. Be aware of these differences:

Cold Symptoms Flu Symptoms
Low or no fever High fever
Sometimes a headache Always a headache
Stuffy, runny nose Clear nose or stuffy nose
Sneezing Sometimes sneezing
Mild, hacking cough Cough, often becoming severe
Slight aches and pains Often severe aches and pains
Mild fatigue Several weeks of fatigue
Sore throat Sometimes a sore throat
Normal energy level Extreme exhaustion


Social networks have tremendous potential to do good in the world. By leveraging existing social connections, people can spread positive health behaviors and attitudes amongst their friends and loved ones.

Ben Reis, PhD, of the Children's Hospital Informatics Program

Flu vaccinations

The flu makes children and adults very ill, causing significant loss of school time and work time, and for children who have chronic diseases like asthma, it can be fatal. The larger percentage of people in the population that get vaccinated, the safer it is for everybody.

Joanne Cox, MD, the medical director of Children's Hospital Primary Care Center