Well, whatever terrain you build a city on gets trashed and becomes a tiles that gives you 2/2/1. So, yeah, I guess that building a city on a lone floodplain tile would get rid of the disease. But you really don't want to do that. A flood plain provides 3 food and 1 commerce initially. It also adds .4 unhealthiness, so I guess you could say it really provides only 2.6 food, 1 commerce after the city grows a few levels. However, if you improve it, you can add another food with a farm, and have it making 4 food (which supports two whole population units, which you can make specialists or have working other tiles) or you could build a cottage on it and have it produce a whole bunch of commerce, support the population working it, and still add one extra food.
Late in the game, they can be quite overpowering. Observe:
3 food, 9 commerce and one hammer per tile, with 6 of those tiles - and those tiles have been like that since about 1200 AD. Because of this city, I maxed out the tech tree before 1800, on Prince difficulty. Yes, all those floodplains give me +2 unhealthiness. Quite frankly, that's nothing, when you consider that those floodtiles also generate 18 base food, 54 base commerce and 6 base hammers. With a market, a grocer, a bank, a library, an observatory, a university, an academy, 3 monastaries... well, you do all the math on what that 54 base commerce quickly becomes in terms of gold and science.
