Influenza viruses Type A, Type B and Type C are the primary cause of the flu, states Healthline. A person infects another person through sneezing and coughing, and these viruses can spread up to 6 feet away. People can also get the viruses from touching an infected object and then touching their mouths, noses or eyes.
A person infected with influenza is infectious between 24 hours before symptoms begin to up to about the fifth day after symptoms present, reports Mayo Clinic. A person who has had influenza in the past may not suffer severely from similar influenza viruses in the future, as the body has already produced antibodies against the viruses. However, these antibodies are ineffective against new influenza subtypes.
Influenza is prevalent in older adults, young children and people living in facilities such as military barracks and nursing homes, says Mayo Clinic. Other risk factors include pregnancy, an impaired immune system, obesity and chronic illnesses, such as heart problems, asthma and diabetes.
Symptoms of the flu include chills, headache, fever and runny or stuffy nose, notes MedicineNet. Children can experience appetite loss, diarrhea, nausea and vomiting. Most adults infected with the viruses take a week or two to recover, but some can develop conditions such as pneumonia.