Images of HPV 16 VLP and HPV 18 VLP (virus-like particle). Gardasil is made of VLPs of four cancer-causing types of HPV (types 6, 11, 16 and 18). A VLP is an "empty shell" of a virus particle that can induce immunity when used as a vaccine. VLPs do not contain viral DNA so they are not infectious and cannot cause disease.

Images courtesy of Merck & Co., Inc. and the National Resource for Automated Molecular Microscopy, The Scripps Research Institute