The EMCDDA collects data on the extent of infectious diseases — primarily HIV, hepatitis C and hepatitis B infection — among people who inject drugs (injecting drug users or IDUs). The data is collected from IDUs each calendar year using two main methods. These are: (a) surveys of IDUs that include serological testing and (b) the monitoring of routine diagnostic testing for HIV, hepatitis C and hepatitis B infection among IDUs. These two sources are the most useful as they can provide data on the proportion of those tested who are infected (i.e. prevalence).