What is the likely cause of leukopenia?

Leukopenia refers to a decrease in the number of white blood cells (leukocytes) in the blood, which can leave the body more vulnerable to infections. One common cause of leukopenia is viral diseases. Viruses can directly infect and damage the bone marrow where white blood cells are produced or they may trigger immune responses that inadvertently lead to the destruction of these cells.

In the context of other causes, while infections can sometimes lead to leukopenia, they more commonly result in leukocytosis (an increase in white blood cell count) as the body responds to fight the infection. Allergies typically involve an increase in certain types of white blood cells, particularly eosinophils, rather than a decrease. Dehydration does not directly influence white blood cell production but can affect blood volume and concentration, potentially leading to misleading lab results rather than a true decrease in white blood cells.

Thus, viral diseases distinctly contribute to leukopenia through mechanisms that impact the production and survival of white blood cells, making this the most appropriate answer.