When I say “sudden onset”, I worked out on Friday and didn’t begin feeling ill until later in the afternoon. The workout was terrible; my legs felt like lead. That happens sometimes, and I didn’t connect it with getting the flu. Instead, I went to the app on my phone for encouragement. According to the stats there [and they don’t tell you what the data pool is, making the number highly suspect], last week I was more active than 86% of Seattle. In the moment, that reinforcement was just was I was looking for.