Microglial nodules are clusters of activated microglia found in the brain, primarily seen in response to viral infections. They represent the brain's innate immune response to infected and damaged neurons. These nodules are especially characteristic of viral encephalitis, where they form around sites of neuronal injury and viral replication. In contrast, bacterial infections typically cause neutrophilic inflammation in meningitis, and the pathology of multiple sclerosis or Alzheimer's disease does not prominently feature microglial nodules. Thus, the presence of microglial nodules is a key histopathological hallmark aiding in the diagnosis of viral encephalitis.

Reference: Robbins Basic Pathology, Kumar, Abbas, Aster, 10th Edition.