I was wondering what are the conditions for getting influenza?
It only happened to me once- While I was trying to make a cultural tour around the map, some Seal tribe villager decided that he would like to arrive in Kaumo lands (in less than 14 days), and asked me to be his guide. Trek back from the mountainous area to the shore, where I left the raft, then paddle back to Driik lands, where at the entrance of the grand river there was my shelter, resupply and follow the river up into Koivula lands where I left the raft (poor guy pretty much swam all the way here, according to Titanic there's not enough space for two on the raft), and faced the harsh weather, and somewhere in Kiesse I've got influenza. A naked night in a sauna did not help in curing it and even made it a bit worse, but after a second visit to a sage and a night in a warm village cabin it was gone. In the end we managed to get to a Kaumo village on time (2 days left for the quest).. Oh, and when I wanted to return I realised that the river froze, so I had Rudolph the Big Reindeer carry it back
So almost two weeks of exhausting travelling in the Dead month (only sleeping when he couldn't properly stand up) might give you influenza, but a night in the sauna and one with 6 more people and one reindeer in the same room might heal it.