On the day you receive cidofovir, blood (about 2 teaspoons) will be drawn 6 times. The first blood draw will occur before the cidofovir dose, and the last blood draw will occur 24 hours (+/- 1 hour) after the start of the dose. This blood will be used for pharmacokinetic (PK) and pharmacodynamic (PD) testing. PK testing measures the amount of study drug in the body at different time points. PD testing is used to look at how the level of study drug in your body may affect the disease.