Article: Synesthesia is considered a rare neurological condition that affects the senses, but it's likely that many people who have it are either undiagnosed or assume that other perceive the world just like them. It is unknown how many people have synesthesia. If you actually see colors in the air, smell, hear, or feel things, you have projected synesthesia. This form of synesthesia is rarer than associated synesthesia and is what people first think of as synesthesia.  Some people with synesthesia (called synesthetes) hear, smell, taste or feel pain in color. Others can taste shapes or perceive written letters and words in different colors. For example, they may see an "F" in red and a "P" in yellow when they read. Some synesthetes see abstract concepts, such as abstract shapes, units of time or mathematical equations floating in space outside their bodies — this is termed "conceptual synesthesia." According to research done in the U.S., there are some factors strongly associated with synesthesia. For example, about 3x more women than men have synesthesia in the U.S. People who have synesthesia are also much more likely to be left-handed and have a 40% chance of a relative having the same condition. Often when people talk of their synesthesia, others assume they are hallucinating or on drugs. What distinguishes true synesthesia experiences from hallucinations is that they are repeatable and predictable, not fanciful and random. For example, if you taste strawberries when you hear a certain song, then one must always trigger the other sensation in a predictable fashion to be considered a synesthete. It doesn't always have to be two-way, though. Synesthetes often note being teased and ridiculed (usually starting in childhood) for describing sensory experiences that others can't experience. Synesthesia is a sort of cross-wiring of the nerves and brain synapses related to the five senses. And no two synesthetes have the exact same wiring scheme. For example, the most common form of synesthesia is grapheme-color, when numbers and letters each have their own color. The colors assigned to each letter are different for everyone, but many people have red A's. Another common form is chromesthesia, or colored hearing — sounds, music or voices that are heard and also trigger the eyes to see colors. However, someone may see the color red whenever they hear the word "dog", whereas another might see the color orange. Synesthetic perceptions are specific to each person.

What is a summary?
Realize that synesthesia is relatively rare but likely under-diagnosed. Know that not everyone with synesthesia experiences it physically. Identify your risk factors for synesthesia. Don't confuse synesthesia with hallucinations. Be aware that no two people with synesthesia have the same experience.