Doubt in week 17 quiz - medical company - lurking variables

Can someone please explain why population per sq km will be lurking variable?

Hj @mehtavatsald02,
For any medic seller, more the area population more will be the sales.
See it this way, It’s a communicable desease, and more the population per unit area, more will be the chances of contracting it, which gives more fear to people living in that area. Hence it will be a lurking variable.