A diseased patient is exposed to an unknown agent while out of the country. The patient’s blood is found to have a high proportion of lymphocytes with CD8 surface proteins in her blood, a likely result of A) the patient having encountered a bacterial infection which elicited CD8+ T cells. B) the disease having been caused by a multicellular parasite, such as can be encountered in polluted water sources. C) the CD8 proteins having been discharged from these lymphocytes to lyse the infected cells. D) a viral infection eliciting proliferation of CD8+ cytotoxic T cells. E) the CD8 proteins having “marked” the surface of cells that accumulate after the infection is over and signal patient recovery.