I played 2 rounds of tournament in total physical and mental exhaustion menatal due to too much study and little rest, physical fall out of shape and did't sleep night before tournamanet- dumb like hell the all scenario but that's not the point. I was playing in positional style 1.d4 and Caro-Kann as Black. I had observation problems and getting recently learned knowledge up a couple of times in the games that caused me loss in both games that could've been won or drawn. I would not play in Positional style if in bad shape, go for 1.e4 and Sicilian, end the game " as quick as you can" but don't go to unclear positions in Sicilian- means if you are Black and play Sicilian play Dragon Accelerated, and if you are White and face Sicilian play side systems like Ne2,g3,c3.
What strategy is best when you're sick?
Online I would use whatever time is alloted for the game to the max. OTB I just wouldn't play Chess if I'm not well.
You should take two pawns and call your doc in the morning.
Seriously, though, I wouldn't play at all if I felt sick, drowsy, depressed or angry since I know I could not concentrate fully. If you really need to indulge in chess, better to read some instructional material until your mind and/or body is fit again to play.
Of course it's best not to play when you're sick and I don't recommend it unless you're already obligated. I'm talking about you serious chess players who have entered a tournament that lasts several days and perhaps you catch a cold in between rounds. Or maybe you promised to play for a team and suddenly the morning of the match you feel sick. You're playing for sure, the question is, should you have a specific strategy taking into account you're not feeling 100%?
Either:
a) don't let your opponent know that you're sick and play drawishly
b) Sneeze on their peices!
I think if you're sick you can still play a good game, unless you feel nausea or something, but for example a cold wouldn't be too bad for playing with. I would just play the same. If for some reason the sickness made me considerably worse I would probably not play a serious game at all.
take vocation time
Sneeze or cough so hard that your opponents king falls down when its his turn, and you will auto-always-win 
just play what u play, but try to simplify the position, dont make anything complicated.
I'd make a point of letting my opponent know I'm not well and proceed to sneeze cough and splutter in the general direction of his face.
Knowing he's probably going to catch it next might be enough to throw him off a bit, or at least bring him down to my sickly level on the day.
I don't know about you guys but the older I get the less impressed I am with people who come into my vicinity with any sort of bug or virus, I feel like slapping them around the ears. They should be quarantined.
Buy a bottle of brandy, go to the table with two glasses and pour oppo one as well - if he takes it you may well equalise the situation, depending on his alcohol tolerance - if he doesn't, chances are it'll make you feel better anyway - an' if it all goes pear-shaped you'll probly be halfway through the bottle by then and won't give a monkey's 
This all raises the question: Has a game ever been postponed/resumed due to illness?
Is there a rule that allows this?
This all raises the question: Has a game ever been postponed/resumed due to illness?
Is there a rule that allows this?
At the higher levels, it has happened quite a bit. I believe it's in their contract or in the FIDE rules. Larsen, Petrosian, Taimanov, and Spassky all postponed their matches with Fischer on account of becoming ill. The media called this mysterious pattern "Fischer Fever." Karpov and Kasparov I believe in their matches have used it from time to time. It occurs pretty much in matches. I'm not aware of a tournament where postponing due to illness has been done.
Always take a Lidl carrier bag. Then you can blow chunks with equanimity. At the mid game adjournment you can offer your opponent some "home-made" coleslaw.
take vocation time
I rather not have to learn the trades throughout the duration of being under the weather.
Tournaments will come and go...I think health is a priority. If you're still fit enough to at least play a game...and not come off like a walking virus, then I guess you just sit at the board and do what comes to you. Or that's what I'd do. No strategy about it.
don't know, every played even when sick and try to simplify is my recommendetion. Happend to me to play a tournament at -1° when I was literally trembling(don't know if it's the right term, google translate told it)and I had a draw whith the one who won the touirnament
My current strategy while ill has been to withdraw from the tournaments and resign from the games I was playing in. It's seriously curtailed my ranking and progress but I was just blundering too many positions and my will alone didn't help keep my concentration and focus. I tried taking a few days vacation but its a longer term thing and felt it was better to let my opponents march on, rather than waiting for me to make moves. IMHO If your not enjoying it and struggling, take a break until you're better again and alternatively study if you can tolerate it. Like poker, chess in my experience is a very psychological game and ill health can both cloud the brain and sap the minds resilience.
Sometimes we have to play when we're a bit under the weather. So should you keep it simple as to not tax your brain with calculations or should you burn your boats and go for an all out attack in an attempt to win the game quickly?