Most Mycobacterium tuberculosis infections are asymptomatic and subclinical infections. In most healthy people primary tuberculosis is asymptomatic, although it may cause fever and pleural effusion. Generally, the only evidence of infection, if any remains, is a tiny, fibrocalcific pulmonary nodule at the site of the infection. Viable organisms may remain dormant in such lesions for decades. If immune defenses are lowered, the infection may be reactivated, producing communicable and potentially life-threatening disease. Testing for latent tuberculosis is done by detecting T-cells specific for or delayed hypersensitivity to M. tuberculosis antigens. This can be detected by either IFN-g release assays (IGRAs) or the tuberculin (purified protein derivative , or Mantoux) skin test.