Parvovirus B19, which kills stem cells. If the patient relies of increased output to maintain their HCT (ex: sickle cell) they can become acutely anemic. Treatment is supportive. AIDS patients can get severe refractory anemia with a parvovirus infection as they do not make antibody against the virus. AIDS patients are seronegative and the diagnosis is made by bone marrow biopsy that shows giant pronormoblasts or blood for PCR for parvovirus. The treatment for AIDS is monthly IVIG.
Acute Clostridia infections can cause a severe hemolytic anemia, by the time you make the diagnosis the patient is dead.
Besides transfusion (duh), depends in the cause.