Mad cow disease, also known as, Bovine spongiform encephalopathy (BSE), is a neurodegenerative disease that happens to cattle.
In the 1980s and '90s, the brain disorder infected 180,000 livestock in Europe and claimed dozens of human lives, devastating the British cattle industry.
It has a long incubation period of 4 years and usually affects adult cattle at its peak age for four to five years. After several researches, it is found that this disease is similar to other diseases such as nvCJD, CJD and scrapie. It believed that the infectious agents could be abnormal proteins called a prion, which carries the disease within a cattle and cause deterioration of the brain and spinal cord. They are found throughout the brains of BSE infected cows, nvCJD victims, CJD victims and scrapie infected sheep. Prion diseases have been known to occur for about 300 years, first occurring in sheep in a condition called scrapie. Prion diseases have now been described in a number of animals, including cows, cats, deer, elk, and others. In humans, it causes Creutzfeldt-Jacob disease (CJD), which occurs spontaneously in 1 in 1 million persons but can also be passed on genetically.

Cattle are supposed to be herbivores; they eat grass or grains in nature. However, they are fed with food containing antibiotics, hormones, pesticides, fertilizers and protein supplements (meat and bone meal) from animal by-products, which lead to a large increase of infectious agents in the cattle feed. As these infectious agents start to spread, more and more cows get this disease which is also likely to spread to humans. Previously normal protein molecules contort by themselves from an alpha helical structure to a beta pleated sheet, which cause the disease. When abnormal prion proteins come in contact with normal proteins, it changes their shape to make them abnormal. These abnormal proteins cannot be used or broken down by the cells, they accumulate into plagues. This accumulation causes the cell to die, releasing the abnormal prions to infect the surrounding cells, repeating the process. Hence this process forms holes in the brain.
Infected cows lose weight and show abnormal behaviour, eventually they will become paralyze, resulting to death. there is no treatment, no cure, and it's 100% fatal.