Whomever suggested that antibiotics are used to treat VIRUSES is incorrect. One of the biggest problems in medicine these days is that people have used antibiotics too much, including attempting to use them against virus infections, an effort that simply does not work. They don't kill viruses, and constant overexposure to antibiotics has allowed many bacteria to evolve resistance.
Amoxicillin, like any of the "cillins," is a broad spectrum antibiotic used to kill bacterial and fungal infectious agents. Thrush is caused by the candida fungus, which is actually present in a large percentage of the population. Thrush is caused when your normal bacterial population gets out of whack and it is then able to develop unchecked.
Unfortunately, antibiotics can help CAUSE thrush, because they often kill other existing, beneficial bacteria that might help fight it off.
One way to help stop this overuse antibiotic problem is to make sure you take ALL of the prescription as advised, even if your symptoms clear. If you don't, what you are doing is possibly helping the bacteria to learn how to adapt...
If it doesn't work, they will switch you to another, newer antibiotic.