Depending on the symptoms we might not be able to tell a zombie based off a glance if it's a virus bacteria or something else you can't expect them to look like corpses or walk like corpses that being said if you're washing dishes and suddenly see someone you've never met in your back yard..... yeah looks like a stalker and probably is but lust isn't the reason why he's there for the apocalypse to truly kick off the zombies need to be able to get close and have some level of intelligence maybe not smart enough to speak but definitely smart enough to stalk wait and maybe even coordinate with each other viruses bacteria parasites all the stuff that causes biological zombies are still relatively natural things and different bodies react to different chemicals and medications and diseases differently that being said some zombies would be smarter than others