Influenza A and B viruses are pathogenic viruses that cause respiratory symptoms in humans. Elderly people are particularly susceptible to pneumonia following infection, making the virus especially significant in Japan, where population aging is rapidly progressing. Influenza A virus can cause pandemics every few decades due to the emergence of novel strains with distinct antigenic properties of their surface proteins. The virus surface protein hemagglutinin begins infection by binding with sialic acid at the terminals of glycans. Progeny viruses budding from infected cells are released extracellularly by neuraminidase, another viral surface glycoprotein, which cleaves terminal sialic acid from glycans. Several clinically used anti-influenza drugs are neuraminidase inhibitors derived from sialic acid analogs, which inhibit virus replication. Glycoscience is an important field for advancing the understanding of influenza virus infection and replication mechanisms, as well as for the development of antiviral drugs.