"Flu season in northern latitudes is from November to March, the coldest months. In southern latitudes, it is from May until September. In the tropics, there is not much flu at all and no real flu season." The initial observation is that flu season occurs during the winter months.
There were four theories posed to explain why we get the flu during the winter:A1: Proximity. During the winter people are closer together (e.g. children are in school) and it is easier to transmit the virus.
A2: Diminished immune response. During the winter there is less sunshine and we don't produce as much vitamin D, hampering our immune system.
A3: Air currents. Air currents in the upper atmosphere during the winter move the virus around more infecting more people.
A4: Virus properties. The flu virus is more stable and stays in the air longer when air is cold and dry.Assume all of these theories have merit, satisfying at least half of the criteria for a scientific theory (it will be required to show this starting with the next set of examples). Also, the theories could explain the initial observation (as described above).
D1: Most of the major epidemics are traced to children.
D2: We don’t see flu in September and October.
D3: People work all year round and crowd into buses and subways and planes no matter what the season.
D4: Transmission of the virus was excellent at 41 degrees. It declined as the temperature rose until, by 86 degrees, the virus was not transmitted at all.
D5: The virus was transmitted best at a low humidity, 20 percent, and not transmitted at all when the humidity reached 80 percent.
D6: The animals also released viruses nearly two days longer at 41 degrees than at a typical room temperature of 68 degrees.
D7: Flu viruses spread through the air.
D8: Flu viruses are more stable in cold air.
D9: Low humidity also helps the virus particles remain in the air. "That is because the viruses float in the air in little respiratory droplets. When the air is humid, those droplets pick up water, grow larger and fall to the ground," instead of infecting other people.
D10: The humidity is much lower in the colder winter months when the air can't hold as much moisture before condensing into water droplets (or snow).
D11: Less vitamin D is produced during the winter months.
Proximity Immune Currents Properties D1 yes no no no D2 no no no yes D3 no no no no D4 no no no yes D5 no no no yes D6 no no no no D7 no no yes yes D8 no no no yes D9 no no no yes D10 no no no yes D11 no yes no no
From this data (Proximity - 1, Diminished immune response - 1, Air currents - 1, Virus properties - 7), it would seem that Virus properties best explain why we get the flu during the winter.