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.
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.