How many viral infections can vaccines prevent ?
Transmission can occur when : body fluids are exchanged during sexual activity , e.g. , HIV ; blood is exchanged by contaminated transfusion or needle sharing , e.g. , hepatitis C ; exchange of saliva by mouth , e.g. , Epstein - Barr virus ; contaminated food or water is ingested , e.g. , norovirus ; aerosols containing virions are inhaled , e.g. , influenza virus ; and insect vectors such as mosquitoes penetrate the skin of a host , e.g. , dengue .