Which of the following diseases is least likely to be viral?

Bubonic plague is the least likely to be classified as a viral disease because it is caused by the bacterium Yersinia pestis. This bacterium is transmitted primarily through flea bites and can also spread through contact with infected animals or their tissues. In contrast, the other listed diseases—Influenza, COVID-19, and Measles—are all caused by viruses. Influenza stems from various influenza viruses, COVID-19 is caused by the SARS-CoV-2 virus, and Measles is caused by the measles virus. This distinction of causative agents is crucial for understanding the nature of the diseases, their transmission methods, and the appropriate treatment options, which differ significantly between bacterial and viral infections.