Vaccinia virus particle, coloured transmission micrograph (TEM). Each particle is composed of a DNA (deoxyribonucleic acid) genome surrounded by a protein coat and lipid envelope. Vaccinia causes cowpox, a disease of cattle and humans that causes skin lesions. This variant (MVA) is a vaccine strain, used to vaccinate against smallpox, a related but more deadly human disease. MVA (Modified Vaccinia Ankara) is an attenuated vaccine, that is it is a live virus that has been altered to be less virulent.