The mycoplasma are a family of bacteria that lack a cell wall and include the smallest known bacteria. Several of them can cause diseases in humans such as atypical pneumonia and inflammatory diseases of the pelvis. Hundreds of others in this group cause diseases in animals. Recent research has shown they are linked to malignant cancer, although the exact connection is not well understood. Because they lack a cell wall, they are not affected by common antibiotics like penicillin and must be treated with broad-spectrum drugs like streptomycin. They can also survive in both oxygen environments and environments without oxygen. Because they are ubiquitous in the environment, they commonly contaminate cell cultures and blood cultures.