PREZISTA™ blocks HIV protease, an enzyme which is needed for HIV to multiply. When used with other anti-HIV medicines, PREZISTA™ may reduce the amount of HIV in your blood (called "viral load") and increase your CD4 (T) cell count. HIV infection destroys CD4 (T) cells, which are important to the immune system. The immune system helps fight infection. Reducing the amount of HIV and increasing the CD4 (T) cell count may improve your immune system, thus reducing risk of death or infections that can happen when your immune system is weak (opportunistic infections). PREZISTA™ is always taken with, and at the same time as, 100mg of ritonavir (Norvir), in combination with other anti-HIV medicines.