Cells are the basic living units of all contemporary organisms with the exception of viruses. In some species of organisms, the body consists of a single cell (single-cell organisms); in other species, i.e. multi-cellular organisms, the cells multiply repeatedly during ontogeny, are variously relocated and diversify until, compared with the microscopic cells, they form the enormous bodies of a multi-cellular organism. The human body is apparently formed of about 50 trillion (i.e. 5 × 1013) cells.