Post-doctoral fellow examining a culture flask containing Madin-Darby Canine Kidney epithelial cells (MDCK), and looking for any signs of growth in a stock of influenza virus. Combining animal cell cultures and recombinant DNA (rDNA) technology, has become a common laboratory practice in the creation of viral vaccines, as well as other biomanufactured products, including hormones, enzymes, and anticancer agents. There are three types of influenza viruses: A, B and C. Influenza A and B viruses cause seasonal epidemics of disease almost every winter in the USA. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.