In general terms, ciguatera is a nerve toxin, and affects everyone differently and sometimes not at all. You cannot build up a tolerance to it. It can be fatal but rarely is. It is geographic, and the area from South Virgin Gorda west to Culebra is where is is quite prevalent locally. The toxin is carried by microorganisms that float free primarily at night, therefore night-time reef predators are most likely to have it. These include kingfish, snappers especially Cubera, large groupers, Hind, and some jacks and barracuda, but especially Horse Eye Jacks. The prevalence among night predators leads to a local saying - "dont eat fish with an eyeball larger than your own "- and others rely on an 'ant test'. Not everyone responds to the toxin the same way. Two people can eat the same fish, and their reactions can be totally different. Local knowledge is key, but it is usually ok if it is from the north side of Tortola, except horse Eye Jacks. They always have it it seems.