Fast foods contain high saturated oils. Saturated oils are oils that are a solid at room temperature. The human body isn't that much warmer than room temperature and so these oils have a potential of building up in our arteries and veins. Heart attacks and strokes are caused by a clogging of the arteries and veins.
It's not that fast food means are as bad as heart attacks - it is one of the causes of heart attacks.
In addition, beef and chicken in fast food restaurants are of the lowest quality. Animals are mass produced and injected with high amounts of GNRH (growth hormones) and many others in order to maximize meat production not to mention they living in putrid cages of their own waste.
Fast food is horrible for your health, but many of us (like me) simply LOOOOVE the taste.