Bacterial Infection: This is often the most common cause of an elevated white blood cell (WBC) count with a predominance of neutrophils and an increase in monocytes. The body's response to bacterial infections typically involves an increase in these cell types to fight off the invading organisms.

Chronic Inflammatory Conditions: Conditions such as rheumatoid arthritis, inflammatory bowel disease (e.g., Crohn's disease, ulcerative colitis), and chronic obstructive pulmonary disease (COPD) can cause an elevation in WBC count, including both neutrophils and monocytes, due to the ongoing inflammatory process.
Viral Infections: Certain viral infections can also lead to an increase in WBC count, although the increase in neutrophils might not be as pronounced as in bacterial infections. Monocytosis can be seen in some viral infections, indicating the body's immune response.
Malignancies: Some cancers, particularly those affecting the blood or bone marrow (like leukemia or lymphoma), can cause an abnormal increase in WBC count, including neutrophils and monocytes.

Sepsis: Although sepsis is a severe condition that might be considered under bacterial infections, it's crucial to highlight it separately due to its high mortality rate if not promptly recognized and treated. Sepsis can cause a significant elevation in WBC count, including neutrophilia and monocytosis.
Endocarditis: Infective endocarditis, an infection of the heart valves, can present with elevated WBC counts, including neutrophils and monocytes, and is a condition that requires early diagnosis to prevent serious complications.
Tuberculosis (TB): TB can cause an increase in monocytes and might also affect neutrophil counts. Given its potential for severe outcomes if untreated, it's essential to consider TB in the differential diagnosis, especially in high-risk populations.

Myeloproliferative Neoplasms (MPNs): These are rare disorders that affect blood cell production in the bone marrow, leading to an overproduction of various blood cells, including neutrophils and monocytes.
Histiocytosis: A group of rare diseases involving abnormal proliferation of histiocytes (part of the monocyte/macrophage lineage), which can lead to an increase in monocytes and potentially affect neutrophil counts.
Leukemoid Reaction: A rare condition that mimics leukemia but is actually a reactive process, often in response to severe infection, inflammation, or other stressors, leading to a significant increase in WBC count, including neutrophils and monocytes.