Demographic changes and the development of transportation contribute to the rapid spread of influenza. Before an idea of a 'person to person' spread appeared, divergent theories were developed to explain influenza epidemics in the past. Intensified virological and serological tests became possible after isolation of the human influenza virus in 1933. The first influenza virus detection methods were based on its isolation in egg embryos or cell lines and on demonstration of the presence of the viral antigens. Molecular biology techniques associated with amplification of RNA improved the quality of tests as well as sensitivity of influenza virus detection in clinical samples. It became possible to detect mixed infections caused by influenza types A and B and to identify the strain of the virus. Development of reliable diagnostic methods enabled fast diagnosis of influenza which is important for choosing an appropriate medical treatment.