You won't have lost your fitness in two weeks, you will just be a little rusty.
The trick I use is not to try and pick up where you left off but to spend the same number of weeks as you had off getting back to where you were. Also be sure that your body is recovered from the bug before you start back or you could be off even longer :(
So you had two weeks off and you run every other day (say 4 times a weeek) so for the first week maybe run something like 2 miles, 2 miles, 3 miles, 2 miles.
Second week, 2 miles, 3 miles, 2 miles and 4 miles. Now you are back to where you were. Hold here for a while till you are ready to move on a little.
You have past the first big hurdle after a layoff, getting back into it so it's all cool running from here and before you know it you will be running further and faster than ever.
Or in other words, you are already DOING Cardiovascular training, you're running ;)
The gym will just give you other options, such as stationary bike, rower, eliptical (sp) etc, none of which should hurt your running.
I would caution however, that depending on your current level of fitness (You mention only running for 4 months) it is possible to overtrain by doing too much. This can make you feel tired and letargic (sp)and affect your running. Your body will still be getting use to your exercise regime, so if you do join a gym start slowly and as is always the case "Listen To Your Body".
Also don't forget you need to fuel your body for the extra work you will be asking it to do.