Briefly describe the cell theory. 

Schleiden and Schwann in 1838-39, formulated the cell theory, which originally contained following two statements
(i) All living beings are made up of cells and products formed by the cells.
(ii) Cells are the structural and functional units of life. 
The cell theory stated by Schleiden and Schwann failed to explain the question of origin of cells. A major expansion of the cell theory was expressed by Virchow in 1855, in his statement ‘Omnis cellula e cellula’ (all cells arise from pre-existing cells).This concept, was the actual idea of Nagelli (1846), which later on was elaborated by Virchow, along with considerable evidences in its support. The work of Nagelli and Virchow established cell division as the central phenomenon in the continuity of life.
Thus, the modern cell theory is based on two facts
(i) all living organisms are composed of cells and products of cell.
(ii) cells are the bosic structural and functional unites of life.
(iii) all cells arise from pre-existing cells. 
Viruses are exception to cell theory as they are not composed of cell. They consist of a nucleic acid (DNA or RNA) surrounded by a protein sheath and are incapable of independent existence, self regulation and self reproduction.