If one accepts the definition of fast food as food based on precooked and reheated ingredients, many of which processed foodstuffs shipped from central locations and served in disposable containers for take-out, then no, there is no real, healthy fast food. Manufacture of these “foods” is not healthy for the workers, the animals or the planet.
For the consumer, however, there are some choices which are moderately healthy in that they are not loaded with empty calories, saturated fats and preservatives. Portion size is another factor that makes fast foods unhealthy. The occasional hamburger with fries and a soda is not going to kill you. A daily diet of double bacon cheeseburgers, super sized fries and a 20oz glass of sugar water will, sooner rather than later.
The healthiest fast food choice is a sub that is made in front of you. You can opt to hold the mayo, and maybe the cheese for a filling and quite nutritious lunch. Tacos are made with fresh ingredients and you can go easy on the sour cream, choosing fresh salsa instead. Many fast food restaurants are offering salads which are great with a low calorie dressing like vinaigrette. Pizza is not a bad choice as long as the crust is fairly thin and you load it with veggie toppings instead of multiple cheeses and sausage and pepperoni and bacon.
Healthy sides include coleslaw, tossed salads and sweet potato fries, especially if you can find somewhere that bakes them instead of deep frying. Ice-cream, smoothies and fruit are good choices for dessert it you feel you need one.
An increasing number of fast food restaurants have nutritional information posted or available. Read this as carefully as you do labels in the supermarket. It is better not to make a habit of eating fast food, but there are options for a reasonably healthy meal if you pick and choose with care.