Coronavirus particles. Coloured transmission electron micrograph (TEM) of several coronavirus particles (blue/orange) with host cells (brown and yellow). The TEM slice has passed through several particles showing the internal core (blue) and the external membrane (orange). The surface proteins (green) embedded in this membrane form a crown (corona) covering the surface (lower centre). The surface proteins are used to attach to host cells and enter them. The particles then use the cell machinery to make copies of themselves (upper right). Coronavirus strains cause diseases such as the common cold and SARS (severe acute respiratory syndrome). Magnification: x38,400 at 10cm high.