Tularemia
By the late 1950s the US biological warfare program was focused mostly on tularemia as a biological agent.  The Schu S4 strain was standardized as Agent UL for use in the M143 bursting spherical bomblet.  It was a lethal biological with an anticipated fatality rate of 40 to 60 percent.  The rate-of-action was around three days, with a duration-of-action of one to three weeks (treated) and two to three months (untreated) with frequent relapses.  UL was streptomycin resistant.  The aerobiological stability of UL was a major concern, being sensitive to sun light, and losing virulence over time after release.