Patients receiving acyclovir as standard of care will be enrolled into this study. They will have blood draws once before they take acyclovir and 10 times after they take the acyclovir (over a total of 12 hours). These patients will be in the hospital already and will not need to make additional trips back to have blood drawn. A total of about 4—5 tablespoons of blood will be drawn for this study. 7 obese patients and 7 matched, non—obese patients will be enrolled into this study.