Short answers, pulled from the story.
Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. There are four types, labeled A, B, C, and D, with influenza A and B responsible for the seasonal epidemics that circulate in humans.
Influenza symptoms begin one to four days after exposure and often include sudden fever, chills, headache, muscle pain, fatigue, sore throat, and a dry cough, with coughing the most common symptom. They typically last two to eight days, and children may also experience nausea, vomiting, and diarrhea.
In a typical year influenza infects 5 to 15 percent of the global population, causes 3 to 5 million severe cases, and is linked to as many as 650,000 respiratory-related deaths. Deaths fall most heavily on young children, the elderly, and people with chronic health conditions.
Influenza viruses, especially influenza A, evolve quickly through antigenic drift, the gradual accumulation of mutations in the haemagglutinin and neuraminidase genes that lets new strains evade existing immunity. Because of this, flu vaccines are rebuilt annually to match the strains in circulation.
Five flu pandemics have occurred since 1900: the Spanish flu from 1918 to 1920, which was the most severe, the Asian flu in 1957, the Hong Kong flu in 1968, the Russian flu in 1977, and the swine flu pandemic in 2009. The Spanish flu, an H1N1 strain, infected an estimated third to half of the world's population.
Mild influenza is treated with supportive measures such as rest, fluids, and anti-fever medication, while severe cases may require antiviral drugs like oseltamivir. Antivirals work best when started within the first 48 hours after symptoms appear.
Influenza spreads mainly through respiratory droplets from coughing and sneezing, which travel less than two meters, as well as through smaller aerosols and contaminated surfaces. Children ages 2 to 17 are considered the primary and most efficient spreaders of the virus.