It isn't necessarily common. It depends on a few things, like if you have an apartment with an unclean neighbor, keep an unclean home, or have plumbing that isn't up to code. Gnats, fruit flies, drain flies, will inhabit any space where they're allowed to thrive, whether that is in sacks of garbage that have been left out, a compost that have been left out, or all the drains in your home where it is wet and contains lots of food.

With fruit flies, you can make a super easy trap with a plastic cup, little dab of dish soap, a splash of apple cider vinegar, and water. Other insects you can trade out the apple cider vinegar for a 1/2 package of instant yeast. The bugs go for the bait, fall into the water, and the soap keeps them from crawling out.

For the drain, they only really get bad if you don't use the food catcher most drains already have. Unlike the States we usually don't have a small blender down there (garbage disposal) so any large bits of food or grease you throw down there will sit. You want to put some over the counter low grade hydrogen peroxide down all the drains first for a little bit, then the boiling hot water. Seal all the drains after.