Spoiler : IMO You can use the lake and the oasis to provide you with fresh water, to prevent having to settle on floods. Bigger thing I see though is that your cap is size 6, and no worker and several unworked tiles. I would have gone cities can work the food first. Cities in middle of large deserts/floods always tend to have health problems. I'd stick cities on each side of desert, or wait until you have a few more food worked. The sickness is just eating all your food, so your making farms, which are only giving 3 food after the sick eats, and not bringing in the commerce.