Just to clarify - a 0.2 um or 0.22um filter will remove bacteria and fungal spores. Larger pore filters (0.45um, 0.8um, 1.0um etc) will not. So there are plenty of micron filters out there that will not filter out bacteria/fungi.
Also - unless you use only new equipment for every step of the injecting process, washing hands and swabbing injection site, there are plenty of opportunities for your shot to get contaminated. Basically - it is incredibly difficult to maintain sterile conditions during injections outside of a medical setting. Filtering is great, and essentinal for safer pill injecting, but it aint the full story.