Why the Flu Dies Out Every Season
Every winter, the flu circulates throughout the general population, getting many people sick. When it comes to diseases, R0 represents the reproduction number, or an average number new cases of an illness each case of an illness will cause. If it is below 1, each case causes less than one new case, meaning the disease will die out eventually. If it is above 1, the disease will keep spreading. In our class, we calculate this by multiplying the chances of infecting a person you come into contact with (p) by the number of people you come into contact with (k). The actual method the CDC and other health organizations use is more complicated, but our method gets the general idea and the result is close enough to the results that the CDC would get to serve the purposes of our class.
The median R0 of the seasonal flu is 1.28. This means that on average, each case of the flu should cause 1.28 more cases. This would mean that the flu should not die out every season, but keep infecting people so it survives forever. Why doesn’t the flu happen year round if its R0 is above 1? Why is it seasonal? Since the flu is seasonal, this must mean something happens during flu season to either the p or the k of the flu. Turns out, both are lowered approaching the end of flu season. As temperatures get colder, flu viruses can stay stable and airborne for longer. The opposite is true as temperatures get warmer. So, as winter ends and temperatures get warmer, flu virus cannot last outside of a body for as long, meaning it comes into contact with less people, lowering the k. As the air gets drier in the winter, which dries up the mucus in the nasal passages, making us more susceptible to getting sick from the flu virus entering our bodies. The opposite is true as we leave winter and the air becomes more moist. Since we become less susceptible, the p lowers. Together, the lowering of the p and the k bring the R0 below 1, meaning that the flu starts to die out. At least until all of this happens again the next year.
https://www.ncbi.nlm.nih.gov/pubmed/25186370
http://content.time.com/time/health/article/0,8599,1914610,00.html