Trademarks: How Different Must Two Marks Be from Each Other

I am new to this, but have a TM infrigement concern. If there is registered mark that shares the same 1st word (which is more descriptive of services/ product) but completely different 2nd word.

Can that still be considered TM infrigement?

Example: Surgery Alerts vs. Surgery Plans

Trademarks: How Different Must Two Marks Be from Each Other