What is a common cause of monocytosis?

Monocytosis, which is an increase in the number of monocytes in the blood, is often associated with chronic inflammatory conditions and infections. Among the options, a viral infection is a well-recognized common cause of monocytosis.

When the body is confronted with a viral infection, the immune system responds by activating various types of white blood cells, including monocytes, which play a crucial role in the immune response. These cells help in phagocytosis, where they engulf and digest pathogens, as well as in the regulation of immune responses. They are particularly effective against viruses and contribute to the chronic inflammatory processes that can result from ongoing viral infections.

In contrast, bacterial infections usually lead to an increase in neutrophils rather than monocytes, while allergic reactions are more likely to cause an increase in eosinophils. Fungal infections can lead to other immune responses but are not typically characterized by monocytosis. Thus, the presence of monocytosis is most directly associated with viral infections, making it the correct choice in this context.