I'll suggest infants get too little UV and too little vitamin D
especially when
they go to solid food. And this results in an over active immune
system.

Good I see you've got PMID references, I'll read them.

You're right on the money.  I've been posting on this topic for years in
sci.life-extension.  I periodically read papers on regulatory T-cells
(FoxP3+ T-cells) in order to map out their inputs and outputs.

Helminths - the intestinal parasites you mention - are not really
parasites but rather poorly adapted symbiotes.  They have wide-ranging
effects on mast cells and immunoglobulins in addition to affecting
Tregs.  I currently take pig whipworm eggs to suppress my autoimmunity.  
It does work.

<http://www.discover.com/issues/sep-93/features/ofparasitesandpo264/>
<http://www.cosmosmagazine.com/node/695>
<www.ovamed.de>

By the way, you do realize the implications of PGE2 having such a
central role for Treg function?  It means every doctor prescribing a
long-term COX-2 inhibitor for rheumatoid arthritis is guilty of
malpractice (especially when you further consider the role PGE2 plays in
mobilizing stem cells to make new cartilage).

Vitamin D3 does more than simply switching on Tregs via the VDR.  Its
downstream product, cathelicidin, has direct wound-healing properties
and antiinflammatory effects on mast cells.  It also needs proper
histone acetylation to pull all this off.

When you scrutinize the actions of NSAIDs and antibiotics coupled with a
low-fiber, sugary American diet, it's easy to see why so many people get
sick.

Tregs:

On signals:  FoxP3 (Neurovax), IL-2 (via CD25), IL-10, TGF-beta, HO-1
(glutamine), alpha-MSH (redheads have a mutation affecting alpha-MSH
binding), Vitamin D3 (also some bile acids via this receptor), PGE2
(COX-2), HSP-60 (acetyl-l-carnitine), androgens (via greater carnitine
transport?), estrogens, DHEA, helminths, HDAC inhibitors (typically via
butyrate from fiber digested by gut bacteria - which requires carnitine
for uptake and metabolization), vasoactive intestinal polypeptide,
probiotics (via butyrate synthesis; loss of butyrate also enhances
fungal overgrowth and chitin production, which can set off allergies and
sinusitis)

Off signals:  mTOR (arginine, leucine; glutamine on the other hand is an
mTOR inhibitor and often given for intestinal support), TNF-alpha, CD8,
IL-21,IL-6, possibly caffeine (via neutral endopeptidase which blocks
VIP)

F.Y.I.  Also requiring histone acetylation are the genes for the mu
opioid receptor and metallothionein, the central metal detoxifier.  I've
posted on this as well.

I've known some very long-lived people who were positively careless
about hygiene and others who were scrupulously clean.

One friend lives on a boat infested with roaches and rarely bothers
washing his hands or cleaning his frying pan or utensils.  He never
brushes his teeth or washes his clothes.  But he's in great condition,
and gets up at the crack of dawn every day and dives into the water to
observe the fish who live under his boat.

A lady I know keeps her house spotless and washes her clothes every day.
 All her pots and kitchen utensils look shiny and new, and she cleans
all her vegetables carefully before cooking them.  She shops for fresh
food daily, too.

This is a very old question, and I doubt it'll ever be answered with a
simple yes or no.

On the other hand, I think it's safe to say that frequent infections and
chronic diseases are generally harmful to your health, and at least some
of the damage they cause is cumulative and irreversible.