All of a sudden this week, we have been inundated with houseflies. 
I'd bet I've killed a hundred of them in a week's time!  Do you have
any idea where they might be coming from and/or how we can get rid of
them.  They are old and lazy (not newly hatched).  We have live in
this house for 21 years and have never experienced this before!
(Previous to this starting we experienced a weekend of extremely heavy
rain...5+ inches)
Thanks for any help/suggestions you can give!

Hi!

Since this is just a comment, you won't be charged for it.

I suggest you start calling a local exterminator service. Usually you
can just clean the house or something but if you are bothered by 100+
flies per week then that is different.

This question lacks sufficient specifics for a specific answer. I
don't know where you are located. You have not said how far into your
local environment you have looked, beyond your own home, to trace the
source

I suggest you enquire of your neighours and your neighbourhood to see
how widespread or else localised the problem is. Maybe it's in your
own home or backyard. Your local public health authorities will
generally be knowledgable and furthermore if the problem is localised
to the actions or activites (eg failing to close bins or leaving piles
of rotting rubbish that is uncovered) of an individual or business in
your neighbourhood they can intervene to resolve the problem, even
prosecute.

If the problem is widespread and on-going then perhaps you're just
going to have to install flyscreens or insect screens on windows and
doors. If they are installed but neglected, holed or poorly fitting
then effect repairs.

The wet weather will probably have contributed but again your local
enviroment is unknown. For example if you live where it rains 50% of
the year then it may just be a coincidence and have no relevance,
however if you are in a desert and this is the first rain in 21 years
then I am not surprised the problem has suddenly arisen. Temperature
is also going to be relevant.

I put the words "fly" "life" and "cycle" together into Google and
found many hits and information about controlling and eliminating
flies. This one
http://www.silcom.com/~vector/flies.html
refers to several species. It is notable that the life cycle - egg to
reproduction stage - varies from species to species but 8 days is
minimum I found so it suggests the eggs were laid 8 or more days ago.

Do you live in the country ?

"Old an lazy, not newly hatched".  My experience with flies suggests
that the slower moving ones are farm/country flies, not the usual
houseflies that are so quick to escape when one tries to slap them.

I suggest using a lot of flypaper and insect spray.

Exactly what I was thinking about.

The rain probably softened dry cow pats or horse dung.

I know of one UK village that gets an annual plague of flies.