When I had food poisoning, my doctor recommended Gatorade G4, which is their low calorie (low sugar) version. You could probably use the calories, but if you can't tolerate the sugar, that might be something to look into. You could use some calories if you can't eat anything, but sugar may cause a yeast overgrowth. You can also try chicken broth, if you tolerate that. That will give you some fluids and some electrolytes (salts). You may want to dilute the broth or get one of the low sodium kinds if the salt is too much.
Yes, Lymies have suppressed immune systems, which makes us get hit harder by anything going around. I have been fortunate enough to avoid most things. On top of that, certain antibiotics can actually weaken your immune system. So, the very thing helping to fight a disease is actually making you more susceptible to getting sick! Fun, isn't it?