Fried food is cooked in hot oil.

This causes the food to have a higher calorie count compared to food that's baked or grilled by comparison. That by itself isn't particularly unhealthy other that helping in weight gain, which has a lot of complications.

The main problem is deep fried food contains a lot of unhealthy fats. Saturated fat and trans fats build up plaque in your arteries that can put you at risk for heart failure, heart attack, and stroke.

Fried food is also commonly purchased at fast food restaurants that serve food with large amounts of sodium, fat, sugar, and few vitamins and minerals. So it isn't part of a healthy diet.

In moderation they are fine, but eating deep fried food constantly is very bad for you in the long term.