biscansplums, a "trojan" is an application hidden inside another one and it is programmed to wreak damage, or replicate itself and spread. Some are supposed to be harmless, yeah right.
It comes from the old story of the Greeks hiding inside a wooden horse, given as a gift, to get themselves into the city to wipe out the residents. The residents pulled it into the city themselves so it's a good description.
Birkenhead........ God's own Room 101.