Certain viruses can agglutinate the red blood cells inside specific animals, which is defined as viral hemagglutination. A hemagglutination assay (HA) is designed to speculate a virus in the tested materials and test whether this reaction is specific. However, the ability of a virus to agglutinate red blood cells can be inhibited by corresponding particular antibodies, which leads to the development of hemagglutination inhibition assay (HAI).