Virus particles and red blood cells, animation. Red blood cells (erythrocytes) contain haemoglobin and carry oxygen and carbon dioxide to and from body tissues. Viruses are non-cellular particles that invade living cells in order to reproduce. Viruses consist of a core of genetic material (a nucleic acid such as DNA) surrounded by a protein coat and surface proteins. Once inside a cell, they use the cell machinery to replicate themselves. The release of the replicates usually destroys the host cell. Viruses cause a wide range of diseases in plants and animals.