Worried about getting "off track"

You will only make yourself worse by exercising while ill. Every gym and workout DVD advise against exercising while unwell for this reason. Rest and continue to eat well and ensure it's good food (not fast/processed food) and you should find you're better in no time.
Diet makes up the majority of weight loss. Exercise is there to help build strength and stamina and tone up etc. So just stick to a reasonable calorie consumption and drink PLENTY of water. Water before each meal will help ensure you don't overeat and mistake thirst for hunger. It will also help you get better sooner as you'll stay hydrated.

I find it's easier to get back on track if I have at least kept to one element of the plan (healthy diet) while ill rather than throw it all away just because I couldn't exercise. x

Of course she was right.. Why would you just blatantly ignore your doctor's advice? How about being more concerned about your general health than getting to a number on the scale. Sorry if that's harsh but this kind of perspective makes no sense to me.

Your body is already working overtime fighting the infection. You stressing it further by eating at a caloric deficit AND trying to do strenous exercise is too much. A few days of rest won't kill you or completely derail your weight loss progress.

No one needs to exercise to lose weight. As long as you eat within your daily calories you'll stay on track. Actually, it wouldn't hurt to eat a couple hundred calories over to help support the extra work your body is currently doing. Even if you're set to lose 1/2 a pound per week, you'll still be at a slight deficit. While you may not see a loss, you shouldn't gain either. Make sure you're drinking tons of water too.