You're speaking of nucleated Red Blood Cells, is that correct? And there is little or no accompanying anemia? Is this an academic question? That would not be a normal finding in a healthy non-anemic person's peripheral blood smear. Nucleated RBC's are very young cells, which would normally be found only in severe anemia or in serious blood disorders.
Eliminating your possible answers in order, the most common RBC parasite would be Malaria, and Giemsa Stain should show up the parasite in RBC's during the febrile phase, but those would not show as nuclei, so parasites could probably be eliminated as a cause. Now, if you tell me that you just came back from a year of missionary work in the Congo or in Guyana, then we might want to look more closely at parasites.
Leukemia is a possibility, and neutropenia can be found in certain varieties of aleukemic leukemia .
If this is more than a hypothetical or academic question, and your last routine CBC with peripheral smear showed nucleated RBCs present, then you should be consulting with a Hematologist for further tests to get a proper diagnosis as to what's causing them to be present. It is also a possible finding after certain drug administration. Suffice it to say that it would be a worrisome finding in an otherwise healthy individual, and should be studied more closely than on a computer question and answer board..