History & Symptoms

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Overview
In humans, influenza's effects are much more severe than those of the common cold, and last longer. Recovery takes about one to two weeks. Influenza, however, can be deadly, especially for the weak, old or chronically ill.

History and symptoms
Symptoms of influenza can start quite suddenly one to two days after infection. Usually the first symptoms are chills or a chilly sensation but fever is also common early in the infection, with body temperatures as high as 39 °C (approximately 103 °F). Many people are so ill that they are confined to bed for several days, with aches and pains throughout their bodies, which are worst in their backs and legs.

Common symptoms of the flu such as fever, headaches, and fatigue come from the huge amounts of proinflammatory cytokines and chemokines (such as interferon or tumor necrosis factor) produced from influenza-infected cells. In contrast to the rhinovirus that causes the common cold, influenza does cause tissue damage, so symptoms are not entirely due to the inflammatory response. Symptoms of influenza may include:


 * Body aches, especially joints and throat
 * Coughing and sneezing
 * Extreme coldness and fever
 * Fatigue
 * Headache
 * Irritated watering eyes
 * Nasal congestion
 * Nausea and vomiting
 * Reddened eyes, skin (especially face), mouth, throat and nose

It can be difficult to distinguish between the common cold and influenza in the early stages of these infections, but usually the symptoms of the flu are more severe than their common-cold equivalents. Research on signs and symptoms of influenza found that the best findings for excluding the diagnosis of influenza were:

Notes to table:
 * Sensitivity is the proportion of people that tested positive of all the positive people tested.
 * Specificity is the proportion of people that tested negative of all the negative people tested.
 * All three findings, especially fever, were less sensitive in patients over 60 years of age.

Since anti-viral drugs are effective in treating influenza if given early (see treatment section, below), it can be important to identify cases early. Of the symptoms listed above, the combinations of findings below can improve diagnostic accuracy. Unfortunately, even combinations of findings are imperfect. However, Bayes Theorem can combine pretest probability with clinical findings to adequately diagnose or exclude influenza in some patients. The pretest probability has a strong seasonal variation; the current prevalence of influenza among patients in the United States receiving sentinel testing is available at the CDC. Using the CDC data, the following table shows how the likelihood of influenza varies with prevalence:

Two decision analysis studies suggest that during local outbreaks of influenza, the prevalence will be over 70% and thus patients with any of the above combinations of symptoms may be treated with neuramidase inhibitors without testing. Even in the absence of a local outbreak, treatment may be justified in the elderly during the influenza season as long as the prevalence is over 15%.

Most people who get influenza will recover in one to two weeks, but others will develop life-threatening complications (such as pneumonia). According to the World Health Organization: "Every winter, tens of millions of people get the flu. Most are home, sick and miserable, for about a week. Some—mostly the elderly—die. We know the world-wide death toll exceeds a few hundred thousand people a year, but even in developed countries the numbers are uncertain, because medical authorities don't usually verify who actually died of influenza and who died of a flu-like illness." Even healthy people can be affected, and serious problems from influenza can happen at any age. People over 50 years old, very young children and people of any age with chronic medical conditions, are more likely to get complications from influenza: such as pneumonia, bronchitis, sinus, and ear infections.

The flu can worsen chronic health problems. People with emphysema, chronic bronchitis or asthma may experience shortness of breath while they have the flu, and influenza may cause worsening of coronary heart disease or congestive heart failure. Smoking is another risk factor associated with more serious disease and increased mortality from influenza.