In recent years, the use of differential equations to describe the dynamics of within-host viral infections, most frequently HIV-1 or Hepatitis B dynamics, has become quite common. Highly active antiretroviral therapy (HAART) has produced dramatic reduction of viral loads in HIV-1-infected children and adults. Soon after HAART was introduced, estimates of decay rates of viral RNA and infected cell populations were obtained using mathematical models, and estimates of the time on treatment required to eradicate viral infection were made. Although the work had enormous impact on identifying the rapid dynamics of within-host viral dynamics, 68" confidence intervals were reported in for the cell-free viral RNA clearance rate since 95" intervals were too large to be meaningful. Mathematical models have had tremendous impact on the field of HIV and viral dynamics in helping to understand the etiology of the disease. Ordinary differential equation models have been used extensively to evaluate a variety of aspects related to viral and immune-response dynamics.