Seasonal influenza is characterized by a

题目：

Seasonal influenza is characterized by a sudden fever, cough, headache, muscle and joint pain, sore throat and runny nose. Most people recover from fever and   1  symptoms within a week without requiring medical attention. But influenza can 2  severe illness or death in people at high risk. The time from infection to illness is about two days.

Yearly influenza epidemics can  3  affect all age groups, but the highest risk of complications occur among children younger than age two, adults age 65 or older, and people of  4  age with certain medical conditions, such as chronic heart, lung, kidney, liver, blood or metabolic diseases, or weakened immune systems.

Seasonal influenza  5  easily and can sweep through schools, nursing homes or businesses and towns. When an  6   person coughs, infected droplets get into the air and another person can breathe them  7   and be exposed. The virus can also be spread by hands infected with the virus.  8   transmission, people should cover their mouth and nose with a tissue when coughing, and  9  their hands regularly.

Drugs for influenza are   10  in some countries and effectively prevent and treat the illness. Some influenza viruses develop  11   to the antiviral medicines, limiting the effectiveness of treatment.

Influenza epidemics occur    12  during autumn and winter in temperate regions. Illnesses result in hospitalizations and deaths mainly among high-risk groups. Worldwide, these annual epidemics result in about three to five million   13    of severe illness, and about 250 000 to 500 000 deaths.

Influenza can cause serious public health and economic problems. In   14   countries, epidemics can result in high levels of worker absenteeism and productivity  15  .   16   most people recover from a bout(轮) of influenza, there are large numbers of people who need hospital treatment and many   17   die from the disease every year.   18    is known about the effects of influenza epidemics in developing countries.

The most    19  way to prevent the disease or severe outcomes from the illness is vaccination. Vaccination is especially important for people at higher risk of serious influenza complications, and for people who live with or care  20   high risk individuals.

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