There is a correlation (negative correlation in fact because patient numbers decrease with an increase in temperature or vice versa) between outside temperature and number of patient visits. There may or may not be causation. Further studies would have to be done to determine this.
There might be a causation factor because lower temperatures is associated with winter which is influenza season.
This means that it is reasonable to assume correlation may mean causation.
If it was let’s say a correlation between sales of ice cream and number of people drowning. There is no causation, as sales of ice cream don’t directly impact number of drowning incidents. It’s due to a hidden variable (temperature).
