Man-made diseases may result from biological weapons in several ways; a disease which is normally initiated by, say, the bite of an infected mosquito, may manifest itself quite differently when caused by inhalation of the pathogen; the natural defences of man towards respiratory infection can be impaired by exposure to lung irritant chemicals, so that the dissemination of a mixed chemical-biological aerosol might lead to signs and symptoms not normally associated with the pathogen used; and confusion may result from simultaneous infection by two or more different pathogens, which may in addition have a synergetic effect upon one another. In addition, new fears arise from the increasing arsenal of genetic engineering techniques which breed strains of well-known pathogens that have increased antibiotic resistance or altered biochemical and immunological characteristics that could ultimately lead to a strain of pathogen so different from its parent that it would be classified as a new disease agent.