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At some random point, your town will catch the Osaka Flu. How you trigger it is unknown. It is uncertain whether it is spread by your friends list or by what, so in essence, it is basically random. There's no way to force it to happen, it will just happen at some point. Having more friends that are active is helpful though. This is because of what happens nce you get it.
Once it triggers and your town has the flu, then you get two new things:
You get the roaming green cloud. It roams around your town and eventually latches onto a tourist (or it may spawn a tourist every few hours, unknown). In any case, when this happens, you can tap that tourist for extra event currency. 14 pins, 7 sky credits.
The new Osaka Flu tab appears. This tab shows you the world map and a status bar labelled "Infection Severity" starting at 0/100. For every pin/shield thing that you clear in your town that was put there by a friend, you will up this number by one. Once you get the flu and you clear buildings, you will see a "+1 Osaka Flu" thing rise off them when you tap them.
Thus, you needs friends to visit you and give you those pins on your buildings, and you need to clear them as much as possible. What's the prize at the end of each of the 5 tracks? Currently, more event currency. That's all.
Thus, to minmax this: a. Get max friends and b. Make sure they're active players who will visit you and give you pins on buildings.
It is possible that there is more to it which is not in the game files yet. The "prize" for each 100 seems awfully low (10 pins, 5 skycredits). The Infection Severity meter goes from 100 on level 1, to 200 on level 2, 300 on 3, but then jumps to 1000 to get the final level. So, maybe it will change as time goes on.

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