At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas molestias excepturi sint occaecati cupiditate non provident, similique sunt in culpa qui officia deserunt mollitia animi, id est laborum et dolorum fuga.
Et harum quidem rerum facilis est et expedita distinctio. Nam libero tempore, cum soluta nobis est eligendi optio cumque nihil impedit quo minus id quod maxime placeat facere possimus, omnis voluptas assumenda est, omnis dolor repellendus.
Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat.
Abnormal proteins called prions are found in brain tissue of diseased cattle and appear to be the particle that transmits the infection. I think this might be what you mean.
This is a very interesting question and provides a great opportunity to learn to use PubMed. This is the online literature base that is the gold standard for scientific literature related to medicine and its may subsubjects. Go to PubMed, search 'madcow disease' then review/causation or etiology.
The two answers given are pretty much right: the 'infectious agent' in mad cow disease is a type of protein called a prion.
Prions are actually normal proteins, misfolded. The protein in its native form is PcP(c) while the protein in its mutant form is called PcP(Sc). When a misfolded PcP(Sc) copy contacts a normally folded PcP(c) copy, it induces the normally folded protein to adopt the misfolded conformation. This happens over and over again; the misfolded proteins all associate with one another to form a growing chain or aggregate.
Although it is unclear how this process causes the disease - some people suggest that it is depletion of the normal protein, other people suggest that it is the presence of these aggregates of misfolded proteins which gum up the cell's inner workings.