A potion of fake illness is Perfect of performance and espionage. You will get the temporary effects that make you seem to appear sick, with out any real lasting disadvantages. 

Your character must make a constitutional check. To determine the fake effects. 

1-6 dice roll: fake vomiting black, green, or pink liquid.  And several square blisters of the color purple and a plenty amount of sweat. 

7-13: basic flu and cold symptoms with a touch of sneezing out what ever the DM wants

14-16: you mysteriously lay a large egg with a lot of fake pain, your hair also turns white. Make them roll a D6 for how many hours this lasts. 

17-20: you completely turn the color green and have a large amount Of diarrhea 

Anything above 20 is the DM choice.