Complex sentence: In some cases, the disease slows or stops if the patient's immune system improves; some AIDS patients with PML have been able to survive for several years, with the advent of highly active antiretroviral therapy (HAART).
Split into simpler sentences: In some cases, the disease slows or stops if the patient's immune system improves. Some AIDS patients with PML have been able to survive for several years, with the advent of highly active antiretroviral therapy (HAART).