GlaxoSmithKline P.L.C. and Pfizer Inc. today announced an unusual deal that will combine their HIV operations into a new company that will account for almost 20 percent of sales of drugs against the virus.

GlaxoSmithKline P.L.C. and Pfizer Inc. today announced an unusual deal that will combine their HIV operations into a new company that will account for almost 20 percent of sales of drugs against the virus.