Commonly people confuse it with simply starving oneself because of the obsession with being skinny.
Anorexia is a form of suicide. Patients have no self-worth, believe the world would be better without them, and are usually quite depressed. It's a rather slow and painful way to die, isn't it?
Once somebody has it, if they get beyond it to have a healthier lifestyle, they will always have a risk of becoming anorexic again. It can take several years to adjust from being anorexic to having a healthy lifestyle. Anyone is susceptible to the disease. Patients being treated for anorexia have 24/7 care.. there is even someone watching them while they sleep in case they try to exercise under their blankets. The most important thing about treatment is to be encouraging and loving.
Starvation kicks in after three days without adequate food. The reason that's the transition from "fasting" is because that's when muscles will start to deteriorate to fuel the body. After an extended period, people will experience vitamin deficiencies which have any number of tolls on the body, weakness to the point of passing out, extreme weight loss, extreme weight gain because the body will swell up and retain everything it can, including water, and if left untreated, it will lead to death. Because of the swelling (which is a lot), a patient only becomes more discouraged and depressed, and the situation worsens.
Introducing food back into someone's body after starvation, called re-feeding, is just as dangerous and deadly as starvation itself, so forcing someone to eat is the worst thing to do.
Often relationships won't change until something drastic happens like extreme weight loss or passing out. Since love and attention is the key to getting through anorexia, many people will be able to act decently among friends if they don't isolate themselves. When it comes to hiding? Some people have special circumstances where nobody would have ever known about the anorexia, even if they were being watched most hours of the day. A common patient may find excuses not to eat out with friends and family, and they may accept food to "save for later" when they'll just throw it out when they're alone. Another thing they may do, such as at home, is take the food to their private area and throw it out.
It's an extremely difficult thing to deal with. Patients have to be loved and given a place in the world. They have to be reassured at all times. It is uncommon for people to get through it without support.