Red blood cells and viruses. Illustration of red blood cells (red) and viruses (spherical, green spikes). Red blood cells (erythrocytes) distribute oxygen to body tissues, and carry waste carbon dioxide back to the lungs. 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 (green). Once inside the 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.