In recent years, trans-fats have risen to the top of the food-and-health agenda. A number of studies have linked trans-fats to coronary heart disease and, as a consequence, food manufacturers, governments and consumers are increasingly concerned about trans-fats: what they are, which foods they appear in and how consumption of them may affect health.
Trans-fats: a brief history

About 100 years ago the discovery was made that liquid oils could be converted to solid fats (which were more useful in food manufacture) by a procedure called hydrogenation. As hydrogen was added to liquid unsaturated oil it gradually became a solid saturated fat (also called a hydrogenated fat).

After the Second World War the process for making hydrogenated and hardened fats from cheaper sources of vegetable oils was widely adopted. Margarines were developed and marketed as alternatives to butter, and vegetable shortenings increasingly replaced the animal fats in cooking.
Trans-fats and health

Trans-fats are actually worse for the health than the saturated animal fats they were designed to replace

As early as 1975 a group of scientists led by Mr Leo Thomas at what is now the University of Glamorgan in South Wales suspected that eating partially hydrogenated fats had a connection with death from coronary heart disease.

The suspected link between the consumption of trans-fats and this illness was subsequently investigated at the Harvard School of Public Health in the US. It is now generally accepted that trans-fats are actually worse for the health than the saturated animal fats they were designed to replace.

http://www.bbc.co.uk/food/food_matters/transfats.shtml