Influenza

Influenza is a viral infection that attacks your respiratory system — your nose, throat and lungs. Influenza, commonly called the flu, is not the same as stomach "flu" viruses that cause diarrhea and vomiting.

Causes and Mode of Transmission

Flu viruses travel through the air in droplets when someone with the infection coughs, sneezes or talks. You can inhale the droplets directly, or you can pick up the germs from an object — such as a telephone or computer keyboard — and then transfer them to your eyes, nose or mouth.

People with the virus are likely contagious from the day or so before symptoms first appear until about five days after symptoms begin, though sometimes people are contagious for as long as 10 days after symptoms appear. Children and people with weakened immune systems may be contagious for a slightly longer time.

Influenza viruses are constantly changing, with new strains appearing regularly. If you've had influenza in the past, your body has already made antibodies to fight that particular strain of the virus. If future influenza viruses are similar to those you've encountered before, either by having the disease or by vaccination, those antibodies may prevent infection or lessen its severity.

But antibodies against flu viruses you've encountered in the past can't protect you from new influenza subtypes that can be very different immunologically from what you had before.

Symptoms

Initially, the flu may seem like a common cold with a runny nose, sneezing and sore throat. But colds usually develop slowly, whereas the flu tends to come on suddenly. And although a cold can be a nuisance, you usually feel much worse with the flu.

Common signs and symptoms of the flu include:

Fever over 100.4 F (38 C)
Aching muscles, especially in your back, arms and legs
Chills and sweats
Headache
Dry, persistent cough
Fatigue and weakness
Nasal congestion
Sore throat

Prevention

Each year's seasonal flu vaccine contains protection from the three or four influenza viruses that are expected to be the most common during that year's flu season. The vaccine is currently available as an injection only. The CDC no longer recommends nasal spray flu vaccinations because during recent flu seasons, the spray has been relatively ineffective.

Controlling the spread of infection

The influenza vaccine isn't 100 percent effective, so it's also important to take measures such as these to reduce the spread of infection:

Wash your hands.
Contain your coughs and sneezes.
Avoid crowds.

Treatment

Initially, the flu may seem like a common cold with a runny nose, sneezing and sore throat. But colds usually develop slowly, whereas the flu tends to come on suddenly. And although a cold can be a nuisance, you usually feel much worse with the flu.

Common signs and symptoms of the flu include:

Fever over 100.4 F (38 C)
Aching muscles, especially in your back, arms and legs
Chills and sweats
Headache
Dry, persistent cough
Fatigue and weakness
Nasal congestion
Sore throat