You know this. Every time you read about character who starts coughing and then brushes it off with “it’s nothing” is dead. Like, super dead. You don’t know if it’s going to be because they have a terminal illness or are about to turn into some insane creature, but you know they’re dead just because of a couple little coughs.
Why this is bad: It is incredibly predictable. It’s probably because it’s one of those things that doesn’t come up unless it’s important; characters don’t randomly start to sneeze or cough like we might sometimes. If it’s included, the readers know immediately it’s for a purpose, so it really takes away any surprise from a character death. We know it from that first, muffled cough and the character reassuring others of their health that they’re doomed, because why else would you have included it?
How you can fix it: Coughing seems to be the go-to thing for writers who want to show that something is wrong with a person since it’s a very obvious action. However, there are tons of symptoms a person can have that are more discrete and can offer a little more surprise. While you mentioning that a character has a weird rash on her arm is definitely going to raise alarm bells, we might not immediately jump to the conclusion that the character is going to die. You can also subvert the trope and use it to the plot’s advantage, especially if you’re working with a world like The Walking Dead where a cough can mean that you might be a mindless meat puppet in a couple of hours, thereby creating drama and tension between the one who coughs and the ones who notice it. Go do some research on different sicknesses and symptoms if you really want to create surprise, as you’ll be surprised what you can show without sending up the black flag of death in the reader’s mind.
Bottom Line: All who cough are not dead.