HIV infection damages the body's ability to resist all types of infection, even by organisms that do not usually cause disease in healthy people (opportunistic infections). Because of increased susceptibility to infection, HIV-positive patients are often advised to take preventive antibiotics on a long-term basis to ward off opportunistic infections. Doctors estimate an individual's level of susceptibility to infection by measuring how many of a particular kind of white blood cell (CD4 lymphocytes) are present in the blood. More CD4 lymphocytes in the blood produce a lower susceptibility to infection. Modern combinations of medications used to treat HIV infection can raise the number of CD4 lymphocytes in the blood. Researchers have already proven that preventive antibiotic therapy can be safely discontinued when the CD4 count rises to protective levels. On the other hand, it is not known whether antibiotic therapy can ever be stopped safely in HIV-positive patients who have already been infected with these opportunistic organisms.