There are a lot of questions here, and karma is a very complex thing.
Changing of seasons does not have too much to do with individual karma, unless one is using the word in a very general way as a shorthand term for "cause and effect" or "interdependent origination", which some, including the person who wrote the article you referenced, would claim should not be done anyway. However, the fact that you were born this time in a place that has cold weather was probably influenced at least in some degree by your behavior in previous lives, at least according to Mahayana writings.
Catching the flu may well have a lot to do with individual karma, as it may depend quite a lot on factors you have some measure of control over, such as your general level of health, going out in terrible weather with inadequate clothing, inattentiveness to your mind-body, or being closely around people who have the flu; cold weather is only a contributing factor. Things are never a simple matter of
either karma
or the law of nature, as your commentator calls it.
Take it from me, though, living in a place that does not ever have cold weather just means a different kind of dukkha.
