What does "Wildfire" mean?

@Richard: the page you link to does mention the connection to Andromeda Strain, but I don't see where it says "Wildfire = The point at which a disease runs out of control"

Wildfire is not established epidemiology terminology. I believe that the precise meaning is thus known only to the episode writer, Glen Mazzara, and whomever he communicated it to. Given that we are more than four years out and the best I can find is an unsourced, unofficial list of possibilities at http://walkingdead.wikia.com/wiki/Episode_Title_Meanings, I don't think there is going to be an authoritative answer until someone asks Mr. Mazzara directly.

I think there are at least two ways to approach this. The first is taking the Wildfire = Andromeda Strain connection at face value. (developed well at http://www.losttv-forum.com/forum/showthread.php?t=78356). In the Andromeda Strain, "Wildfire" was a pre-existing government biohazard containment protocol that was activated when a pathogen of alien origin arrived on earth. In this context, "day 194 since Wildfire was declared" would mean 194 days since the government noted the presence of the zombie pathogen and activated its containment protocols. Presumably these were secret at first so the general public did not know about what was going on. "63 days since the disease abruptly went global" would then indicate that is when the government officially lost control of containment and due to multiple outbreaks worldwide the disease became public knowledge, which would have been shortly after Rick was shot and went into a coma. I think the strongest evidence for this is that Jenner is transmitting as "WILDFIRE MSB3417 ACTIVE" which implies he is part of an Andromeda-style containment team.

A less likely but equally interesting possibility is that "Wildfire" is not the containment team but rather the pathogen itself. Infectious diseases have two relevant features: contagiousness (how fast they spread) and virulence (how strongly they affect the host). The most dangerous epidemic diseases are high contagiousness, high virulence. The English colloquial term "spreading like wildfire" would imply the pathogen has a high contagiousness, as does the fact that Jenner tells Rick that everyone in his group has been infected. Possibly wildfire also refers to the virulence, since we know that one of the symptoms of the disease killing its host is high fever (as we saw with Jim in the episode). In this case "day 194 since Wildfire was declared" would mean something like 194 days since the pathogen was discovered and named or 194 days since Patient 0 contracted it.

In this case c. 130 days of spread without the public knowing about it does seem odd. Note that the CDC's "Edwin Jenner" is an obvious reference to Edward Jenner, the pioneer of the smallpox vaccine. Note that the RW Jenner used pus from a different disease, cowpox, to create his smallpox vaccination. And note that the WD Jenner said the CDC facility was designed to contain "weaponized smallpox". Put these together and it may be implied that Wildfire began as a government bioweapons program, attempting to change one disease into something else. In this case, the "day 194 since Wildfire was declared" may have been the time since the disease was created in secret, and the 63 days is roughly the time since it escaped from government labs into the general population.

You apparently aren't a long time fan of Science Fiction, or you would have instantly picked up the reference and homage to Michael Crichton's "The Andromeda Strain" in both the episode title and the repeated use of the word "Wildfire."

Wildfire was a declared state of pathogen/biological emergency in "The Andromeda Strain" defining a situation where positive control of a pathogen with the capability to destroy civilization as it's known was lost.