Sherlock is in Scotland Yard's lab, checking some evidence for a case in the microscope. While he's busy looking at it his assistant walks in, late

"You're late, come here, hand me my phone"

he says without looking up from the microscope, when his assistant walks over to him and hands him the microscope he finally looks up

"Thank y-"

He frowns slightly in what could be concern as he looks at his assistant, who he just realized looks rather sick

"you're sick"