close up of white mask stack and word tb in red crosss on blackground

allergy asthma background bird black blackground borne breathing caucasian clean close control cross crosss danger defense disease doctor ebola epidemic epidemiology equipment filter freezone h1n1 h5n1 haze hazy health hospital infection influenza mask mers nurse object pandemic particle pathogen pollution prevention protect protection protective sick stack swine transmission tuberculosis unhealthy virus white wood wooden word