Most phobias, including Emetophobia, usually develop from specific childhood experiences that severely traumatized the individual. When a young child experience a traumatic event their brain takes action to prevent that traumatic situation from being encountered again in the future by imbuing them with a fear of those circumstances. In the case of Emetophobia, things related to an instance of vomit become feared.
The act of vomiting itself may not have been what was traumatic about the experience. Rather, the conditions surrounding the act of vomit may have been what was traumatic. For example, a child who threw up in the midst of a very large thunderstorm may associate the loud noise, and maybe lack of electricity, with the vomit. This will lead to a fear of vomit, although it was not the vomit itself that was traumatic.
In addition, parents can be a large factor in the development of Emetophobia in children. Children who observe their parents fear of vomit may develop a similar fear, as they have no role model to demonstrate to them that vomit is a regular part of life. While Emetophobia is not hereditary genetically, many children of emetophobes develop the same fear.
For other people, Emetophobia could stem from something quite the opposite. It has been suggested that an abnormal lack of vomiting as a child could lead to the development of Emetophobia. The thought process behind this is that when most kids learn to cope with vomit at a young age, the individual who rarely or never vomits as a child never learns that vomiting is a normal part of life. As they grow older, they may begin to fear vomit because it is unknown to them.
The most important thing to learn from this, is that Emetophobia is a learned behavior, as opposed to a condition that you are genetically predisposed to. It is because Emetophobia is a learned behavior that you are also able to cure Emetophobia. Remember that!