The Stars In Shroud: Gregory Benford

In the future, mankind's Japanese-dominated empire comes into conflict with the mysterious alien Quarn. The Quarn will attack without provocation and will destroy themselves rather than be captured. Suddenly, the invasion stops and a lone Earth ship finds a remote space colony in the throes of a strange contagious mass psychosis - evidently of Quarn origin. The ships captain realises the danger if the plague victims reach Earth. Then his command is suddenly overruled and the fate of mankind seems sealed with inevitable doom. Cover art by Tony Roberts.