Transmission and Illness
How is H1N1 flu transmitted (spread)?
Health officials and scientists believe that H1N1 flu spreads in the same way as seasonal flu. When people cough or sneeze, droplets (tiny drops) of respiratory secretions are pushed out of their mouth or nose and into the air around them. When people have the flu, these droplets have flu virus in them. People who are nearby may swallow or breathe in the droplets. This may lead to infection and the flu.
Infected droplets may land on others' hands or be on the hands of someone with H1N1 flu. When people with the virus on their hands touch their mouth or nose, this can lead to infection and illness.
Infection may also occur when people touch surfaces that have flu virus on them and then touch their mouth or nose and accidentally swallow or breathe in the virus.
How long can people with H1N1 flu spread it to others?
People with H1N1 flu may be infectious (contagious) from 1 day before they have symptoms until 24 hours after their fever is gone. Some individuals, including children (especially younger children) and immunocompromised persons, may be contagious for longer periods.
How long after catching H1N1 flu do symptoms appear?
Health officials believe that the incubation period for H1N1 flu is similar to seasonal influenza. Flu symptoms typically start 2 to 3 days after infection, but may begin as soon as 1 day and up to 7 days after infection.
What are symptoms of H1N1 flu?
The symptoms of H1N1 flu in people are similar to the symptoms of seasonal flu. The main symptoms of H1N1 flu are:
- Fever of 100°F (37.8°C) or higher (measured with a thermometer)
- Cough
- Sore throat
- Chills
- Headache
- Rhinorrhea (runny nose)
- Shortness of breath
- Myalgia (muscle aches and pains)
- Fatigue (weariness, tiredness, or lack of energy)
- Nausea, abdominal pain, vomiting, or diarrhea (these are not typical seasonal flu symptoms)
As with seasonal flu, people at high risk for flu complications may not develop usual flu-like symptoms (for example, they may not have a fever because their immune system is weak and can not mount a response to fight off the virus). Contact your health care provider if you have questions about your symptoms.
At this time, most people in the U.S. with possible or confirmed H1N1 flu have had mild illness that has resolved on its own or with over-the-counter medications, such as fever or pain relievers (for example, acetaminophen, aspirin*, or ibuprofen).
* Never give children or young people under 19 years old aspirin or aspirin-containing products unless told to do so by a health care provider. These include any product that contains aspirin, acetylsalicylic (uh-SEET-l-sal-uh-SIL-ik) acid, or subsalicylates (sub-sa-LIS-a-lates). For fever, use a different nonprescription anti-fever medication (such as acetaminophen or ibuprofen) instead.
Do not give children under 4 years old over-the-counter cold medications unless your health care provider tells you to.
Who is at high risk of developing complications from H1N1 flu?
Currently, people assumed to be at high risk for H1N1 flu complications include:
- Children younger than 5 years old (especially those under 2 years old)
- Pregnant women
- People (including children) with:
- Lung conditions (especially asthma)
- Heart conditions (except hypertension)
- Chronic kidney and liver disease
- Blood disorders (including sickle cell disease)
- Neurologic disorders including:
- intellectual and developmental disability
- cerebral palsy
- spinal cord injuries
- seizure disorders
- other neuromuscular disorders
- Metabolic disorders (such as diabetes)
- Immunosuppression (including those who have weakened immune systems from conditions such as cancer or HIV infection or from medications)
- Children and adolescents (younger than 19 years old) who are on long-term aspirin therapy and who may be at risk for developing Reye syndrome* after influenza virus infection
- People 65 years or older
- Residents of nursing homes and other chronic-care facilities
As health officials and researchers learn more about H1N1 flu, these categories may change.
*Reye syndrome is a very rare, but serious illness. It is most common in children and adolescents. Studies have shown that taking aspirin increases the risk of getting it. Never give children aspirin unless told to do so by a health care provider. For more information on Reye syndrome, visit the Medline Plus Reye syndrome page.
What complications can flu cause?
Some people are at high risk of developing serious complications if they get the flu. Some of the complications that flu can cause include:
- Worsening of chronic medical conditions, such as:
- Asthma
- Congestive heart failure (CHF)
- Diabetes
- Pneumonia
- Bronchitis
- Dehydration
- Sinus problems and ear infections (especially in children)
People who develop flu complications can become very sick and even die. That's why it's so important for people at high risk of flu complications to contact their health care providers if they have flu-like symptoms or have close contact with someone with flu-like symptoms.
When should someone with influenza, including H1N1 flu, get emergency medical care?
If you, or someone you know, is ill and has any of the following symptoms, seek emergency medical care immediately.
In children, symptoms that require emergency medical attention include
- Fast breathing or trouble breathing
- Bluish or gray skin color
- Not drinking enough fluids
- Severe or persistent vomiting
- Not waking up or not interacting
- Being so irritable that the child does not want to be held
- Flu-like symptoms improve but then return with fever and worse cough
In adults, symptoms that require emergency medical attention include
- Difficulty breathing or shortness of breath
- Pain or pressure in the chest or abdomen
- Sudden dizziness
- Confusion
- Severe or persistent vomiting
- Flu-like symptoms improve but then return with fever and worse cough
Does anyone have immunity to H1N1 flu?
Because H1N1 flu is new, many people have little or no immunity (protection) against it. This is why it has spread so quickly and widely.
While most people who get seasonal flu are 65 years and older, currently most people who get H1N1 flu are between 5 and 24 years old. Early reports indicate that no children and few adults younger than 60 years old have immunity to H1N1 flu.
However, some adults over 60 years old may have antibodies against H1N1 flu. Antibodies fight off foreign substances (such as viruses, bacteria, fungi, or parasites) that enter the body and can cause illness. However, it is currently unknown how much protection, if any, people with these antibodies have.
