one of "flusurvnet", "eip", "ihsp", or two
digit state abbreviation for an individual site. Exceptions are
New York - Albany ("nya") and New York - Rochester
("nyr")
age_group
a vector of age groups to pull data for. Possible values are:
"overall", "0-4y", "5-17y, "18-49y,
"50-64y, "65+y".
years
a vector of years to retrieve data for (i.e. 2014 for CDC
flu season 2014-2015). Default value is the current year and all
years values should be >= 2009