How do you know you have the flu?

Vaccination programs against H1N1 swine flu are under way in the United States, China and Australia and will begin soon in parts of Europe.

As people await their chance for immunization, below are some questions and answers about flu symptoms and what to do if they arrive before the vaccine does.

There are also online self-assessments to help determine whether someone has the flu, including www.flu.gov/evaluation/ from the U.S. government and h1n1.cloudapp.net/Default.aspx from Microsoft Corp.

WHAT IS INFLUENZA AND WHAT ARE THE SYMPTOMS OF H1N1 SWINE FLU?

Influenza is a virus that infects the nose, throat and lungs. Seasonal flu typically kills 250,000 to 500,000 people globally, mostly the elderly, but also very young children, pregnant women and people with chronic diseases.

H1N1 swine flu is a new strain that appeared in March and became pandemic in June. Like seasonal flu, H1N1 is usually mild and requires no medical care. But H1N1 also differs from seasonal flu because it is more likely to infect children and young people than the elderly.

Most H1N1 symptoms are the same as seasonal flu: fever, coughing or sore throat, runny or stuffy nose, headaches, body aches, chills and fatigue. But swine flu also can cause vomiting and diarrhea.

Dozens of other viruses cause similar symptoms, but one hallmark of influenza is a sudden onset of symptoms. An illness that develops gradually is likely to be from another virus.

WHAT IF SOMEONE GETS SICK?

People who are infected with the H1N1 virus may be contagious as early as one day before they show symptoms.

Because H1N1 is the overwhelming flu strain circulating globally, health authorities say anyone with influenza should assume it is the swine flu.

Quick flu tests may not detect H1N1, so doctors are advised not to bother testing people with flu-like symptoms.

The U.S. Centers for Disease Control and Prevention recommends that people with H1N1 stay home from work, school, travel, shopping, social events and public gatherings until at least 24 hours after the fever has disappeared.

Officials also urge the sick to avoid contact with anyone in a high-risk group, including pregnant women, children and infants, and people with chronic medical conditions including asthma, diabetes or heart disease.

In the meantime, officials recommend frequent hand-washing, covering coughs and sneezes, and in some cases use of face masks to avoid spreading infection.

The main remedies for mild illness are rest and ample fluids such as water, broth, sports drinks or electrolyte beverages made to prevent dehydration in small children.