Final answer:
Prophylaxis against Pneumocystis pneumonia, caused by Pneumocystis jirovecii, is with the drug trimethoprim-sulfamethoxazole (TMP/SMZ).
Step-by-step explanation:
The prophylaxis against the causative bug of Pneumocystis pneumonia (PCP), which is the fungus Pneumocystis jirovecii, is typically with the combination drug trimethoprim-sulfamethoxazole (TMP/SMZ), commonly referred to as Bactrim or Septra. This medication is effective in preventing and treating PCP in patients who are immunocompromised, such as those with AIDS or taking high dose immunosuppressants. While sulfa drugs can have adverse effects, they are generally considered the best treatment option due to the severity of untreated PCP infections, which can be often fatal.