So your giving your characters, and the society they live in, the freedom to conjure and create, but only at the risk of attracting a magivore which will take their magical abilities away. Sort of like a "get out of jail free" card that you might only be able to use once. That elevates the choosing of which crime to commit, to a very interesting level. I'm using the word "crime" here with a little artistic license. I don't want to imply that using magic is in any way morally wrong.
Two schools of thought are obvious...
There will be some who use their magic immediately, to jump ahead of
the competition, hoping to create the seeds of a prosperous life and
set themselves up before they loose the ability. These people are gambling on their ability survive and thrive after the magic is gone.
Others will hold onto their magic to use as a last resort. These
people are gambling on their ability to find prosperity without
magical assistance and pessimistically, they are trying to stay
prepared for the inevitable challenge that only magic can solve.
Both attitudes have merit, but since your magivores spread like a common cold, the second type of people are gambling against their likelihood of catching that cold and loosing the magic without ever getting any benefit from it.
Magic is out in the open, accepted by the society and free from secrecy. Contracting a Magivore infestation would probably be comparable to getting leprosy. Once your magic falters, people wouldn't want to get near you anymore. There would be isolated colonies for contaminated people and social customs would evolve to test for contamination before getting to close to a stranger. I didn't know either of the approaching men, so I gestured a rune in the air between us. The glowing lines of my magic shone in the air between us, demanding that they respond. They each gestured in kind, and the light they caste was free of flicker or fault. Confident in our mutual safety, we dropped our runes and entered the room together.
Spreading it: You've suggested that magivores are like bacteria, but I would find it more interesting if they were larger, maybe mouse size, invisible and intangible flying creatures. Their invisibility, intangibility and flight capabilities might be the very reason that they need to eat magic, to keep their defenses working.
If they were flying ghost-mice, you could still have your characters and their society thinking that they are a disease. It might just happen that glass, plastic and latex can block their intangibility, so normal anti-bacterial defenses would also work against them. Until somebody caught one in its magic-starved, defenseless state, nobody would know that they were an infestation rather than a infection.
Sterilizing it: Personally, I would leave the culture impotent in their attempts to cure the "sickness". Since they fundamentally misunderstand the nature of their enemy, everything they have tried has failed. Magivore infection, like leprosy or a social disease, would be seen as a permanent, non-fatal condition, with a slight chance of remission, but no hope for a cure. This would free you to use your new creation in some very real social commentary. Any fiction you wrote about the world where these creatures live could easily be crafted into a metaphor of our world, with magic representing sex and the magivores playing all the dark roles from VD to Aids. Such metaphors are precious and rare, so you should make use of them when you find them.