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.