I have a cat with herpes and it helped her a little, but it made her so
drowsy.  I also have used it for a cat with skin allergies.

The ones on the faucets are usually way too small and ineffective. The
filters I use are the standard and fairly large, like a quart bottle.
Sears still sells them although I don't know if they still sell the
housings. GE now sells a lot but they are a little smallers. Home Depot
has GE. I found that almost all the big names used the same size
filters, Ametek / US Filter / Plymouth / Pentek / Culligan / Kleen Plus
/ American Plumber / Bruner / Many Others. The filters might be
different. I have a double filter housing but I needed to plumb it into
my pipes under the sink. It's not difficult but it's something you
could do if you're careful. It's probably trivial for any plumber to
do. I tap into a pipe and voila, just about.

I'm finding it harder to find the simple biggish housings that were
universal size for almost any filter made. You can even get bigger
housings that can do a whole house, whose filter is like a half-gallon
almost.

I prefer an undersink one with its own little faucet so it lasts a long
time and I am not wasting water on washing dishes. Just for drinking
and cooking. THe really good filters, at least one with 0.5 micron
openings, will slow the water down so another reason to have it
separate where I don't care if it takes a minute or two to fill a
gallon jug.

These big filters are usually good for even gasoline sutff, MTBE, for
about 6 months or 750 gallons. If that's not a concern, they can last
for a year or over 1000 gallons of water. Those little things on the
faucet never seems to go much beyond 20 gallons. They are just too
tiny.

But before you do anything, ask for a copy of the annual water report
from your municipal authority. They usually test for parasites. You can
discuss this with them, your water manager. This report is usually a
freebie and in the smaller towns, it's often sent for free. In the big
cities, you have to ask for it since it might have some info you don't
want to know about :)

This stuff improves the quality of the water. It's a good compromise
before the next step which are three filter units with reverse osmosis.
That requires more hassles but is pretty good. Now distillation is yet
another thing.

But first get your water report so you know what baddies, if any, to
get rid of. You might be lucky and your water authority does a superb
job like using ultraviolet which is wonderful, no bad side products
like what happens when using chlorine or chloramine, which is the more
common way for water public purification systems.

I could go on about this forever. I bought my Sears / Ametek unit
decades ago based on Consumers Report. It has a meter so I know how
much water is being filtered. And it's the old-fashioned universal
size, biggish, so the new and old and residential and industrial
filters all can fit inside. It's not that big really. About the size of
a briefcase.