They may look pretty and provide a little flavor, but those pieces of sour citrus can spread germs. In tests, 70% of samples had organisms on them, some as many as 25.
Your stomach acids can neutralize most of these, but some can cause tummy trouble. If you have diabetes, have had kidney failure, or get chemotherapy, you could be more likely to be affected by them.
Either way, if you can, it's a good idea to see if the servers use gloves or tongs to garnish drinks before you order yours.