Many children go through a picky eating phase, but there are some who don't grow out of it, and carry their picky eating habits into adulthood. In most cases, these adult picky eaters will only eat bland, white foods, like bread, chicken, plain noodles, cheese sandwiches, etc. They will almost never eat fruits or vegetables. As a result, they tend to be underweight, although sometimes they can appear completely healthy.
I personally know two people who have what is now being called Selective Eating Disorder. The number of foods they will eat can be counted on two hands, and they will reject everything else without trying it. (One of them is in his '40s and had never eaten a strawberry until last year.) Social situations where food is involved fill them with anxiety, because they know they will likely be asked why they're not eating anything. But the thought of trying something new literally terrifies them. Actually attempting to try a new food makes them gag, or sometimes even vomit.
Studies have suggested a link between extreme picky eating and OCD, autism, and/or Sensory Processing Disorder. In some cases, a person might be a "super taster", for whom strong flavors are intensified and can be overwhelming.
I wish I knew how to help my friends who have this problem, because not only are they physically unhealthy, but they recognize that it severely limits their social lives. But how do you help someone who is terrified of food?