Currently it is unclear when the NORS Outbreak data starts, only that it ends in 2021.

[simonleandergrimm]

https://github.com/naobservatory/p2ra/blob/00ec297987190b3260e25ba3f4ea36905164a648/pathogens/norovirus.py#LL79C1-L94C41

[jeffkaufman]

The data goes back a long way, but the older data includes fewer states so naively comparing outbreak numbers won't work. HISTORY_START holds the first year we're using.

[simonleandergrimm]

Done with #140