Why can it pop up all of the sudden?Sort of like with people, anything that "compromises" the immune system can cause this to pop out. Of course puppies just don't have a mature immune system anyway, but also there's lots of things that occur IN puppyhood that exacerbate this.
One of the first 'predictable' things that can trigger an outbreak is vaccination. Now, of course, vaccines are totally necessary! But have you been in the situation where it's been easier and less expensive to you take the dog to the vet (particularly that 'first visit") and get everything done at once? I mean I'm frugal and busy and it used to be my usual thought that it was silly to incur more than one office visit (not only costly but often a scheduling nightmare for us humans as well)???? Well, actually there are really GOOD reasons "why not".
For a moment, think about what a 'vaccine' is. According to Encarta, Vaccination (also called 'immunization') is a preventive medicine. It's how people/animals are made "immune to an infectious organism by inoculating them with a form of the organism that does not cause severe disease but does provoke formation of protective antibodies."
So either by using live virus or 'killed' virus, a little bit of this 'disease' is injected into the patient, which SHOULD trigger the bloodstream to build a defense against this. But, when you give a bunch of shots all at once, the immune system has to develop antibodies to EVERYTHING all at once and you may well overload the animal's system. In Callie-English it's like there's only so much strength to battle everything at once, so while it battles ONE thing, something else is left uncovered and other areas become weaker and susceptible to problems!!!
How Should vaccinations be handled then?Be careful. Don't do too much at once.
Particularly, I've found Muffin breaks out every time I get him vaccinated. At this point he's been vaccinated more than once with all the parvo, etc. shots and my vet wants to call a bit of a moratorium on it - but I stress he HAS received those shots. In Florida I HAVE to do a rabies vaccine every year - but I've learned to do it in cooler weather and when there isn't a lot else going ON in our lives. I know it's gonna stress him out and I'll have to probably dip him - so I'm not going to plan it when I know we're going to be gone a lot or if some traumatic event has just happened or is about to.
This leads me to the next "trigger". Stress. And honestly, this becomes an umbrella covering just about anything. It's not surprising that "stress" is a contributor here. Think about us humans! When are WE prone to getting sick? When we are tired, stressed out and maxxed!!! Same with a dog!