Can you lose activity over time?
You can't lose them unless you got your posts removed and your posts count became lower then your activity.
Once your post(s) for a selected period(s) are deleted then you will lose activity.
For example, someone having 42 Activity (junior member) and 42 posts. He made only 1 post in his first period then this post is deleted by a mod, at final he will have 41 posts and 28 activity (he will become a newbie again).
Conclusion : make many constructive posts per period in non useless topics.
Wait, so if you make a post that get deleted, do you just lose 1 activity or do you lose all activity for that 2-week time period?