Synesthesia is perceiving a sense impression when another sense is stimulated. The commonest one is perceiving letters as having colors; the test for this is to have the person write down his color and then later ask him to do it again, because ordinary people can't do it over the course of weeks, but a synesthete's last for years.
It may be that babies can't sort out sense impressions, and synesthetes just don't develop the same sorting mechanisms as the rest of us. It does tend to weaken or even vanish as the synesthete grows up. Not always, and then, since it doesn't interfere with living, it's not a disease.
The author is a synesthete herself and discusses her and many others' experience. Observing that for her "O" is white, and that's just as much a part of it as its roundness; Z has bubbles in it. She also has another trait of seeing time as physical and having color.
And the uses that composers and authors and artists have made of their synesthesia. Including one where a musician, a non-synesthete, could identify the compositions from paintings of musical compositions.
Much interesting stuff.