As the name suggests, Cell Wall Deficient Bacteria (CWDB) are a group of bacteria - originally identified in 1935 - which lack a normal cell wall, e.g. Bacillus subtilis.
Some research groups have suggested that CWDB might cause diseases in humans and other animals, but so far the experimental evidence is lacking.
A review study from 2005 found that :
The true relevance of cell wall-deficient bacteria (CWDB) to disease is unknown
Source : The Journal of International Medical Research 33: 1 – 20
Note: It's recently been found that 'normal' bacteria such as E.Coli can be converted into CWDB by a simple enzyme reaction which removes the cell wall.