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Influenza: Risk Factors and Diagnosis

Influenza viruses A and B cause acute respiratory infection. Influenza may present with symptoms similar to the common cold (see Upper Respiratory Infection), but also often causes more severe systemic symptoms, such as high fever, myalgias, weakness, and severe pulmonary involvement.

Symptoms may begin abruptly after a 1- to 4-day incubation. However, an infected person can pass the virus to others before symptoms begin and for approximately 1 week after symptom onset. Uncomplicated influenza is self-limiting. In high-risk populations, however, influenza can cause significant morbidity and mortality.

Influenza has become a matter of increasing concern due to the recent outbreaks of H5N1 avian influenza and the confirmation that the pandemic of 1918, which killed 40 million to 50 million people, was caused by an avian virus with properties similar to H5N1.1,2 Wild birds may carry influenza viruses in their digestive tracts and are believed to pass them to domesticated birds, typically in poultry farms, where viruses may replicate and be transmitted to humans. H5N1 has proven fatal to nearly 100% of infected chickens and about 50% of infected humans.3

Risk Factors

Contact with infected individuals. Direct contact with persons who have an upper respiratory infection permits viral transfer. Coughing or sneezing aerosolizes respiratory droplets containing influenza virus. The droplets commonly make contact with hands and household surfaces and can be easily transmitted to uninfected persons. They can also be directly inhaled. Saliva, however, is not an effective mode of transmission.

Closed settings. Homes and schools have higher transmission rates, compared with typical work settings.

Immunocompromise. Persons with compromised immune systems, including those with malnutrition, diabetes, and chronic respiratory disease, generally have a higher risk of mortality if they are infected by influenza. However, the influenza pandemic of 1918 and the recent H5N1 outbreaks have led to a high rate of mortality in young, otherwise healthy persons.

Winter season. Influenza infections more commonly occur in the winter, but cold climates are not necessarily a risk factor for disease occurrence or severity.4

Contact with infected birds. Risk for H5N1 influenza is principally related to contact with infected domesticated birds or bird feces, secretions, and products.


Influenza typically has few physical findings and, in mild cases, may be indistinguishable from common colds.

Patients with symptoms or signs of lower-respiratory infection, such as dyspnea and rales, should be evaluated for pneumonia or exacerbation of chronic lung disease. Persons who appear seriously ill may require hospitalization and antibiotic treatment when bacterial pneumonia or systemic infection is suspected.

Rapid influenza tests that distinguish influenza A and B are valuable diagnostic tools when influenza is suspected in the clinic and when antiviral therapy could shorten the course and reduce symptoms. Note: Rapid tests should not be used during outbreaks, because the probability of flu is high and testing is not cost-effective. Individuals who present with flu-like illnesses during an outbreak are treated accordingly without further testing. Clinical judgment is paramount in these situations.

Cultures help track the specific strains of viruses circulating in a certain region or during a particular season.


Influenza: Prevention and Treatment >>