Unfortunately, bad fats are a food additive found in most fast foods and all processed foods. Most are man made compounds that have been used since the 1950's which coincides with the start of the ever increasing occurrence of heart disease.
Man-made trans fats are created by modifying vegetable oil by turning them into artificially hydrogenated oils. They are found in margarine and shortening which are used to create processed food, junk foods, and to deep fry various dishes.
The process to hydrogenate oils uses high heat, pressure and a chemical solvent like hexane. The resulting compound is bleached and deodorized. The strange part is that the FDA allows a small percentage of the solvent to remain in the oil.
Hydrogenated vegetable oils cause inflammation which signals your body to deposit cholesterol on artery walls as a healing agent. Clogging your arteries is the cause of heart disease. We have been eating these oils for more than 60 years in our society. We are just now starting to realize how bad they can be. Food labels are now stating if trans fats are in the food. Some countries are banning their use. or setting a date for phasing them out.
As hydrogenated vegetable oil is being phased out in some locations, companies are turning to other types of oils that are still processed in the same way. Sources now include soybean, cottonseed, and corn oils. Unfortunately, these processed oils still cause the same effects though not as drastic in some cases. Somewhat safer but still dangerous.
Your best bet for avoiding these trans fats is to stay away from fast and processed foods and eat whole, natural, minimally processed foods.
There are bad man-made trans fats but there are also good ones too. Natural trans fats are made by animals like cattle, sheep, and goats. As they are created, they are stored in layers of fat the animals create. As a result, any product coming from these animals will contains them as well like their milk and meat. The highest quantities of healthy fats are found in animals that have been fed natural grass and allowed to roam free as opposed to being fed soy and grain on farms.
Weight loss experts have been using a common natural trans fat called conjugated linoleic acid (CLA). The man-made version has been made into pills but the source is a plant based oil and the process used is similar to the one used to create hydrogenated oils. Natural sources in ruminant animals is far better quality. There are healthy grass-fed beef and other free range meats available online.
Food labels list trans fats but understand what you are eating to determine the good ones versus the bad ones. Meat or dairy product will most likely contain the natural good trans fats while the processed foods will contain the bad ones.
One more important note: Food labels can be tricky to understand as "trans fat free" does NOT mean it has no fats. The lower limit of what is allowed is 0.5 grams. It has trans fats but is still labelled fat free.
These are a number of good resources I would recommend:
Nourishing Traditions - by Sally Fallon and Dr. Mary Enig
Eat Fat, Lose Fat - by Sally Fallon and Dr. Mary Enig
An article about saturated fat truths by Dr. Mary Enig
Trans fats are not all bad. Understanding the products you are eating and whether the food label is listing good or bad fats is a good starting point. Eating whole foods is a really good idea, especially if you can eat organic.