Composite of two negative transmission electron micrograph (TEM) images of highly magnified spots of two forms of monkeypox virus. On the left is an example of the C-form, or capsular form, of a monkeypox virus particle, and on the right is an M-form, or mulberry form of the virus. These two brick-shaped particles were discovered in cell culture. The surface of the M-form virion is covered with short, whorled filaments, while the C-form virion is spot-penetrated and appears as a dense, well-defined core surrounded by several stratified areas of different densities. , Inger K. Damon and Sherif R. Zaki, 2003