Gotta say, I'm not a fan of the "hygiene hypothesis." The following quoted passage summarizes, very succinctly and clearly, some of the main problems I have with the idea that exposure to a less hygienic environment may be beneficial:
"In the elderly a chronic basal systemic inflammation prevails - which is evident by enhanced CRP or IL-6 plasma concentrations - and by compromised defense mechanisms against invading microbes. These alterations belong to the physiological ageing process of the immune system (immunosenescence) and are regarded as an inflammatory response towards lifelong antigen stress ("inflammatory/pathogen burden"). This lifelong antigen stress evokes an age-dependent basal inflammatory activation of innate immunity as well as a wasting of specific immunity: it is supposed that in the course of life-time due to a multitude of infectious/inflammatory events ("multiple hits") an inflammatory stress prevails or "inflammatory/pathogen burden" accumulates, which substantially contributes to an enhancement of the inflammatory parameters of natural immune response. Such enhanced inflammatory parameters characterize persons at increased risk of degenerative diseases like atherosclerosis or coronary heart disease. The risk is the higher, the higher the "pathogen burden"."
from: The Role of the "Inflammatory/ Pathogen Burden" for Cardiac Ageing (AntiCardAgeing)http://clinicaltrials.gov/ct2/show/NCT01045512
In short, inflammation is one of the central weapons in the immune system's arsenal. The problem is that inflammation, as well as inflicting damage on pathogens/antigens, also appears to do damage upon the body itself. It's akin perhaps, to a country being invaded, and as the country's military is fighting off the invader, there is a lot of unavoidable self-inflicted collateral damage to their own people and infrastructure.
The immune system in humans is composed of an adaptive/specific and innate component.
The adaptive component can learn about a particular pathogen/antigen, then develop specialized specific responses to it. According to what I've read, and although it may be rather early to be certain about this, it appears that there is only a limited total number of adaptions that can be made. So, for argument's sake, 100 adaptions might be the total number, and with each new novel pathogen/antigen exposure another adaption is used up until the adaptive ability is exhausted. This may be one reason why the elderly can have a much weaker immune system, and even die from normally minor illness/infection. This may also be why immunizations becomes less effective in old age.
The innate component of the immune system, on the other hand, has a number of "stock" reactions that it uses to try to defend the body. These reaction are, at least relatively, non-specific and not specialized for the pathogen/antigen they are used against. One of these reactions is inflammation. There is evidence that with each engagement with an antigen/pathogen the basal widespread state of inflammation increases, until eventually a constant significant widescale inflammatory state arises. This inflammation has been implicated in many of the most serious and common chronic diseases of aging.
I think it's also relevant to consider that during the last 200 years(and certainly continuing during the last 20 years) there has been ever-increasing levels of hygiene(at least in the developed countries), and at the same time ever increasing levels of health and longevity. Further, there seems to be almost universal consensus(not a small thing considering we're talking about medical sciences) that increased hygiene has played one of the most important roles in increased human longevity and health during that period.
Taking these factors into account then one could, I think, reasonably hypothesize
that reduced lifetime exposure to pathogens/antigens/germs(possibly with some important caveats) may generally be the most beneficial course to take.