It is systemic. It needs the right conditions to flower - which typically happen in flower (as you might notice your veg room often looks clean and then bam in flower they go and it shows). It is like herpes except you can cure it with diligence or if you're willing to use a harsh fungicide - which IMO is worth it but just make sure you're taking clones off that clone you sprayed and then clones from the clone you took from the original
Hope that wasn't too confusing. It has a pretty long half life (Eagle 20/Nova 40 and others like it).
I've had luck using UV-C light to treat but it requires insane diligence. I'm pretty sure it kills it but on big plants it's not super practical and you wind up missing spots. On clones I think it will work and it's what I'm currently using - it seems to be working.
I'd go as far to say as most plants in most peoples gardens around here probably have it either latent or full blown. You really can't see though if it's been infected.
Also there are many types of mildew. Depending on the region the conditions they enjoy vary. The kind that's common around here likes cool and damp. I believe there are types in the midwest that prefer dry heat. Which of course makes some sense, right?
It's worth looking at your local farmers websites to see what they have to say about it. The Canadian FDA had a website I used to have bookmarked on my old computer breaking down the different types that exist and the conditions they like.
You can get it tested to to see what you're dealing with which can be a huge help in solving the problem.
One other small - I've heard of people having great luck dealing with by spraying their A/C with Eagle20/Nova 40 as you can imagine the spores really collect. Me personally? I'd rather use bleach for that purpose. I think cleaning your A/C is a pretty big deal though as well as your intakes and room in general.