sudden death illness

A mild illness that comes very soon after another mild illness. For example, when you have a flu and about a week before you had strep throat. The reason it's called sudden death is because it's the second round of your illness problems, as it were, and also it makes a morbid pun.

She had a sudden death illness that day, and she was sort of bummed, because that day she'd missed a big potluck at the community center.