As yet the cause of this cluster of infections is unknown but all those infected had undergone a lumbar epidural steroid injection.
Aspergillosis infections are not passed from person to person (unlike meningitis caused by viral infection (viral meningitis) or bacteria (bacterial meningitis)) so that can be ruled out as a cause. More likely routes of infection are use of a non-sterile batch of instruments/medical solutions, badly contaminated environment where the procedure was carried out or poor procedure/human error.
This type of infection is extremely rare amongst people who do not have a compromised (suppressed) immune system as there is no 'way in' for the infecting fungus to exploit in order to infect - our skin and highly effective immune systems usually prevent infection even when hundreds of fungal spores are breathed deep into our lungs.
However once past the barrier of our skin the fungus has a much better chance of surviving within our bodies (though it is still only a very slim chance) so doctors take precautions to prevent infection such as ensuring the injection site is sterile and obviously the needles, catheters and solutions used are sterile. If either becomes contaminated there is a small chance that infecting spores can be pushed deep into our bodies.
In these cases there is an added risk factor as the steroid used can aid infection if spores have been introduced into the injection site - they tend to locally suppress immune response.
The outbreak was detected by vigilant doctors quickly and correctly diagnosing this infection which is difficult to positively identify as it mimics other types of infection. Further infections have undoubtedly been prevented by their professionalism.
Both clinics are closed until the cause has been detected.
Further Information & References
Aspergillus Website article