This is a good idea: in a search engine’s query logs, look for outbreaks of queries like [[flu symptoms]] in a given region. I’ve heard (from Roddy) that this trick also works well on Facebook statuses (e.g. “Feeling crappy this morning, think I just got the flu”).
For an example with a publicly available data feed, these queries works decently well on Twitter search:
[[ flu -shot -google ]] (high recall)
[[ "muscle aches" flu -shot ]] (high precision)
The “muscle aches” query is too sparse and the general query is too noisy, but you could imagine some more tricks to clean it up, then train a classifier, etc. With a bit more work it looks like geolocation information can be had out of the Twitter search API.