Fast food as a group of foods is normally unhealthy so your answer would be "it can't" but you can eat foods that are healthy and don't take a long time to prepare, but they probably won't be found in fast food outlets. Fresh food is fast and healthy.
Bread, rolls, flatbread is ok if not taken to excess, e.g. one sandwich for lunch is fine but healthiness depends on its filling. Avoid mayonnaise and sauces. Roast meats rather than processed (ham, pies) is best, cheese in moderation, tuna without sauce, and sardines are very good for you.
Since fast food is cheap, it is consumed by many people. It is also readily available. However, fast food is usually fried and does not always have the vitamins and minerals you need. Many fast food options on the menu are made with preservatives and contain many calories.
However, over the past ten years or so, these chains have started to add healthier options to their menus. They received a backlash from the healthcare industry as well as from health insurance companies about how fast food restaurants are contributing to health problems. Fast food, though, can be good for you if you are willing to make the right choices. Salads, grilled chicken and light dressings are always good options. Fruits and oatmeal are good too.