Your sponges work hard for you, scrubbing dishes, dirty countertops, and errant spills all around the kitchen. They also create a haven for the germs you wipe up with them, even if you rinse them after use. If you've ever had a sponge go slightly stinky (or outright smelly), you know exactly what I'm talking about.
Think about it... if you wipe up germs with your sponge and then wipe things with said sponge, you could just be transferring those germs from one spot to another, which... yuck.
There are a few things you need to do to get this in check. First, most people keep their sponges for far too long. Sponges should be replaced every couple of weeks, which is why we're such fans of sponge multipacks. And you should definitely sanitize them at least weekly (and possibly even more frequently — I do mine every couple of days) by running them through the dishwasher or soaking them in a bleach solution of ¾ cup bleach to 1 gallon of water.
Helpful Hack
If you're out of bleach, zapping your sponge in the microwave will work, too. Soak up about a half-cup of water with your sponge, and then microwave it on high for one minute if it's a scrubby sponge or two minutes if it's cellulose.