Generally, granite countertops should be quite sanitary. Only when they are poorly maintained do you run into issues. Granite itself is porous as a stone, but less so than other natural stone countertops (e.g. marble). They can be even less porous depending on the finish you go for also: a polished finish will have less pores. And beyond this, granite has most of those pores filled with sealant. Typically, you want to reseal annually. But you can always test this by letting a little water sit on the countertop for 15-30 minutes. If it looks like the water is being absorbed into the rock, then it’s time to reseal. When it comes to resealing, you can do it yourself or hire an expert to do it for you.
The dangers of not resealing your granite countertops is that they may start to stain and absorb other liquids or bacteria. If those pores aren’t filled with sealant, they can be filled by other substances, which may make them more unsanitary if untreated. If that is a concern to you at all, you may want to consider quartz countertops instead. They need no sealing ever because they are naturally non-porous.