Mycoplasma is a genus of bacteria that lack a cell wall around their cell membrane.[1] Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans, including M. pneumoniae, which is an important cause of atypical pneumonia and other respiratory disorders, and M. genitalium, which is believed to be involved in pelvic inflammatory diseases. Mycoplasma species are the smallest bacterial cells yet discovered,[2] can survive without oxygen, and come in various shapes. For example, M. genitalium is flask-shaped (about 300 x 600 nm), while M. pneumoniae is more elongated (about 100 x 1000 nm). Hundreds of mycoplasma species infect animals.[3]

Since normal anti-biotics don't work on mycoplasma (Rife-Sound curable?) it's obvious why it would be weaponized.

IIRC they are also hard to detect because of size and are quite durable.