Mycoplasma (Gallisepticum and Synoviae) is a bacteria not a virus. Both can affect the kidneys and hens are not routinely vaccinated to prevent them catching it. Infected birds become carriers, remaining infectious for life, although some birds may become immune.
Should new birds be brought into your flock?
Ideally no, unless the infected birds can be kept well away from new birds and the owner is prepared to use ‘barrier nursing’ (change clothes / boot dip between the two groups). Some birds die, some birds are only carriers and some birds are sick and recover, but repeatedly have bouts of illness (not normally as bad as the first time though).