There's no perfect answer here, though it's a question clients fire at me often. After having been knocked down the last few days I had plenty of time on my hands to feel this one out.
The age old rules when it comes to workouts are: if your symptoms are above the neck (i.e. head cold) you can probably still move forward, slowly and carefully, with both light cardio and strength moves. Don't try to pound through your HIIT, rather give your body a bit of light resistance work and do some separate light cardio. The cardio is great for your lungs, your cells, and for sweating out your virus. Make sure you're well hydrated before you begin, and be diligent about re-hydrating afterwards. Listen to your body, and if you get dizzy, it's time to stop.
If you have body aches and/or fever, it's better for you to rest. This is frustrating for those of us who like to feel in control (!) because we have to simply sit there, feel it, let it have us, and wait for it to go away. The one thing that you can do here is sip lots of ice water, icy homemade smoothies with as much nutrition in them as you can manage, have some warm baths and sweat it out in your sleep. As for sleep - get lots of it.
Try to still get some protein in. Mix it in your smoothies, have some broth, buckwheat, soups, stews, etc. Add as much colour into your day of meals as you can. You want those extra vitamins to help your body kick the bug.
A couple days off the gym won't kill you, and the best part is that once that virus has left you, the power of relativity will kick in and you'll be more motivated than ever to feel strong and healthy again.
Sometimes when we feel on top of things, life tosses these things our way to force us to be powerless again. Sometimes it comes with a purpose. Be easy on yourself, be patient, and let it run its course. Maybe you needed to remember who the boss was? :) Above all else, have great respect for your body as its complex system works hard to deal with what's thrown its way. Science is pretty cool, at the end of the day.
One more little bitter additive, let's hope this mean weather learns where the door is in the coming days. Right?
Be well! xo