Wellll, not exactly. Selmak died from Jacob's old age / disease. Selmak had deliberately stayed in Jacob far past the point of diminishing returns when it comes to restoring the host's body to good health, because he had important work that needed doing which prevented him from investing his efforts finding a new host. Selmak and Jacob both knew that Selmak needed a new host cuz Jacob was dying, but they agreed to ignore this fact in favour of finishing their work... effectively sacrificing Selmak in the process. When it finally came to light, it was too late, as Selmak no longer had enough strength to change hosts.
I must be misremembering the order of things in Reckoning/Threads. I thought for sure that it was Selmak who was dying, but Jacob kept Selmak around long enough for him to finish his work defeating the Replicators/Go'a'uld.
EDIT: Wikipedia, the cardinal source of all knowledge(tm), confirms my memory:
In "Threads", Jacob reveals that Selmak is dying of old age after living for 2,000 years. During the events of "Reckoning", Jacob insists that Selmak hold on so as to complete their calibration of the Dakara superweapon to defeat the Replicators. As a result of this effort, Selmak slips into a coma and can no longer exert the necessary will to save Jacob's life when he dies. Jacob tells Samantha of his intention to die with Selmak, and that he considers his years spent with him as a gift. Jacob passes away shortly after Selmak in the SGC.