The rationale is that by providing a safe setting in which to use substances, that's a clean setting with clean equipment, you reduce transmission in several ways. For example, I've been involved in a tuberculosis transmission that was through nasal use of substances. It's not just about injection. Any access to substances can lead to the transmission of infectious diseases.
If you've ever seen the environments of addicts who don't have the ability to access a clean setting to use the substances that they're addicted to use, those settings are ideal for the transmission of all sorts of infectious diseases.
It's sometimes described as a syndemic, where this group in the population who are very vulnerable, very poor and have poor nutrition are mixing together often with other people who are using. They often have been subject to all sorts of disadvantages, which means they may not be vaccinated against recent childhood immunizations, for example. Therefore, there is a list of outbreaks that are linked to the use of substances, including outbreaks of diphtheria, for example, and the spread of antimicrobial resistance. They are all linked to that environment in which the uses occur.
