A virus, such as the
Influenza virus, or any other harmful virus, weakens the body. The body becomes more susceptible to
bacterial infections. Symptoms such as nausea, coughing, strep throat, and
nasal congestion are actually bacterial infections that occur quite often after a
viral infection. Secondary infections can also occur after a
primary bacterial infection as well. Not to cause any confusion, that bacteria is perfectly capable of infecting the host without a virus present, but the virus does increase it's likelihood to stick around.
FYI, Streptaccoci pneumonia, which causes strep throat, pneumonia, and other respiratory infections, was originally called H. Influenza because flu used to be believed to be caused by bacteria. After the virus was discovered, the name "Influenza" was transferred to virus.