Complex sentence: In those with HIV/AIDS and a CD4 count of less than 200 cells/uL the antibiotic trimethoprim/sulfamethoxazole decreases the risk of "Pneumocystis pneumonia" and may also be useful for prevention in those who are immunocomprised but do not have HIV.
Split into simpler sentences: In those with HIV/AIDS and a CD4 count of less than 200 cells/uL the antibiotic trimethoprim/sulfamethoxazole decrease the risk of "Pneumocystis pneumonia". This antibiotic may also be useful for prevention in those who are immunocomprised but do not have HIV.