Double Helix: Double or Nothing — Peter David

Like a lethal bacillus coursing through the bloodstream of the galaxy, a fiendish master plan has spread death and terror across the 24th century -- until two heroic captains risk everything to stamp out the evil at its source!
For more than a decade, since the very beginning of the Starship Enterprise'sâ"¢ mission, unknown enemies have unleashed a string of deadly biological weapons throughout the Alpha Quadrant, inflicting ghastly epidemics upon the Romulans, the Cardassians, the Bajorans, and even the Federation. Now, at long last, Captain Jean-Luc Picard joins forces with Captain Mackenzie Calhoun of the Starship Excalibur to track down and confront the hidden architects of the plagues -- and avenge the deaths of thousands!