So I've been sick for about a week now and today I thought about that one fic with sick ! Ten.  So using the "work on something else" note for a bit of this. 

Donna was frantically searching the kitchen: still a bit thrown off by his request for some celery,  Emptying cabinets and hunting in crispers for what felt like forever, she finally found half a stalk of celery. 

"This what you were looking for?" she asked, bringing it to him on a tray along with a steaming mug of tea. 

The Doctor sneezed. "Oh Donna you're brilliant!" he said, experimentally eating a bit of the celery. All of a sudden, his face started to turn an odd color... almost purple. 

Donna looked on, a bit in shock and assuming it made him worse. 

"Don't worry, Spaceman" she said, trying to seem reassuring though deep down she was near panic mode, "I'll sort you out. " 

The redhead began trying everything she could find in the medical kit- aspirin, antihistamines, cough drops, even a strange-looking herbal supplement. The Doctor refused just about everything before falling asleep and beginning to even snore a little. 

Oh this was just wizard now, Donna thought.  

"What am I supposed to do?!" she shouted to the TARDIS. "You know,  he says you're alive- not sure how that works, but a hint would be nice right now. " 

As if in answer, the screen that had displayed the Doctor's temperature now displayed a picture of the tree they'd seen earlier. 

"I have to go back to that tree?" Donna asked, "And what, get a leaf or something?"

The picture zoomed in to show exactly that.