Force India driver Paul Di Resta is battling a throat virus and a fever ahead of Sunday's Japanese Grand Prix.
The Scot, who finished 12th in qualifying, picked up the illness before travelling to Suzuka.
"I'm not 100%," he said. "It's something in my throat, my tonsils and I'm going through cold sweats. It's a bit of a fever.
"By all means it's not ideal and the last place you would want it is here when you've two races on the bounce."
Drivers will go straight from Japan to South Korea for next week's race.
"At the moment, I'm not recovering and I don't know if it's at its worst yet, but it's not easy and this circuit is one of the more physical places as well," Di Resta said.
"I've been eating more than normal and trying to fill myself with fluids to keep my strength and sugar levels going - and it's hot here this week, so that's not helping."
Di Resta finished one place and a third of a second behind team-mate Adrian Sutil in qualifying but remains confident of a points finish on Sunday.
"I've struggled to dial the car into how I like it and I think there is going to be a lot of tyre degradation tomorrow, as high as we've had anywhere," added Di Resta.
"We made changes overnight to prevent it as much as possible, but only time will tell as to what will happen.
"We saw today a number of cars in Q3 opting not to run to save the tyres.
"Looking at it, if we get in the points then we'll be very happy, particularly as we've not had the pace other people have had this weekend.
"So maximising any potential opportunity will be the key."