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For most people, the word “flu” means just about any short-term illness that lays them low. To other people, flu means any short-term illness that makes them cough, feel sick and achy, and have a fever. However, to doctors, flu means an illness caused by an influenza virus. In this report, the word “flu” means an illness caused by an influenza virus.

The early symptoms of all kinds of flu are similar, and they also are similar to the symptoms caused by infections from many other viruses. That makes it difficult to diagnose any kind of flu by symptoms alone.

All flu viruses spread by jumping from one host (person or animal) to the next mainly in small droplets of saliva and mucus or in feces. Inside each droplet of saliva can be tens of millions of tiny flu viruses. Once a virus finds a new host and infects the new host’s cells, it begins to reproduce quietly, making millions of copies of itself before causing symptoms. This time between when the infection begins and when symptoms start is known as the incubation period. With the new swine flu virus, the incubation period is between 3-5 days.

Symptoms of ordinary flu

Ordinary human flu tends to occur during colder weather, typically between November and March in the United States and elsewhere in the northern hemisphere. Flu-like illnesses that occur in other months are less likely to be caused by an influenza virus. Although most people with flu have a cough and sore throat, these symptoms can be caused by many other infections, as well, even during flu season.

When someone develops a cough and sore throat in flu season, there are several things that tend to distinguish flu from another kind of infection. A person with influenza is more likely to experience the following:

  • symptoms start abruptly—over just a few hours
  • feel very sick (no energy for anything)
  • high fevers (100° F to 105° F) that rise to this level rapidly, in the first 12 to 24 hours
  • bad headaches, aching muscles, aching joints, pain on moving the eyes, and discomfort in bright light—along with the cough and sore throat

In other words, as anyone who has had it can attest, “ordinary” flu can make you feel pretty sick for a while. However, in healthy people, ordinary flu subsides once the human body mounts a defensive response: healthy people usually return to full health after about a week. For older people or those with diseases of the heart or lungs, adding the burden of serious lung infection to their existing condition can be too much. In fact, 30,000 people die each year in the United States because of ordinary human flu.

Symptoms of H1N1 flu

The initial symptoms of this new flu have been similar to the symptoms of the regular flu. Nausea, vomiting, and diarrhea may be more common.

Here are the differences so far between H1N1 flu and seasonal flu:

  • H1N1flu seem to thrive in much warmer than the usual cold weather when seasonal flu thrives.
  • Symptoms seem to be no worse than those of seasonal flu, although that may change when the weather gets colder in late fall and winter.

People with flu symptoms should seek immediate medical attention if, instead of recovering, they become sicker with the danger signs listed here.

For adults, the most worrisome symptoms are these:

  • shortness of breath
  • persistent vomiting
  • confusion
  • dizziness

For young children, the most worrisome symptoms are:

  • very rapid breathing
  • not interacting normally, not eating or drinking normally, being unusually irritable, or appearing unusually sleepy
  • high fever and rash
  • a bluish color of the lips and skin

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