The cell theory states that all living organisms are composed of cells and that all cells arise from pre-existing cells. While this theory is largely accepted, there are two notable exceptions:

1. Viruses: Viruses are not considered living organisms because they lack the characteristics of life, such as the ability to reproduce independently. They are essentially packets of genetic material (DNA or RNA) enclosed in a protein coat. Viruses require a host cell to replicate, which makes them reliant on the cellular machinery of the host.

2. Prions: Prions are infectious agents composed solely of protein. They are misfolded proteins that can cause other proteins to misfold, leading to the formation of aggregates that can damage cells and tissues. Unlike viruses, prions don't contain any genetic material and are not considered to be alive.

While viruses and prions don't fit neatly into the traditional definition of cells, they do highlight the complexity of life and the limitations of simple definitions.