A Plague of Sorcerers by Mary Frances Zambreno, 1991.

Jermyn Graves comes from a family of sorcerers, but although he seems to have the talent, it takes him unusually long to get his familiar. Familiars, animals with a special link to a wizard, assisting them in their magic, come to their masters when they are ready to begin learning magic, and without one, Jermyn cannot really be a wizard.

When Jermyn’s familiar finally appears, everyone is surprised to find that it is a skunk. Some people make fun of him for having such a strange familiar. No one is quite sure what the meaning of his familiar is, and the exact nature of Jermyn’s magical talent is still unknown. Still, his aunt apprentices him to a theoretician, a man who does not actually practice magic himself, but who studies the theory behind it. He can assist Jermyn until Jermyn’s true talents become known and he can study with a master who shares his specialty.

Jermyn likes his new master and the young orphan girl who lives with him. However, disaster soon strikes the city. Sorcerers are falling victim to a strange disease that sends them into a coma. Despite their best efforts, none of the remaining sorcerers can discover the source of the disease or its cure, and all the time, there are less and less sorcerers to continue the work. Time is running out, and Jermyn and his unusual familiar may be the only ones capable of finding the answer.

Jermyn does have a special talent which he comes to fully understand, and he uses it to save the other sorcerers and put an end to the mysterious plague.  Although the others had laughed at Jermyn’s skunk, he later comes to realize that she was the one keeping him from getting the plague himself because of some unusual qualities of her own.  The combination of mystery and fantasy is exciting, and there is a mysterious villain who may not even be aware of their villainy because of their own hidden sides.

This book is currently out of print, but there are used copies still available, and you can also buy an electronic copy from American Fantasy Press.

There is a note in the beginning of the book that says that the first two chapters are based on a short story written by the author earlier.  The book is currently available online through Internet Archive.  There is also a sequel to this book, Journeyman Wizard.