Cat flue is a very serious disease in cats indeed and yes it can be deadly
I'm afraid, but mostly to weaker cats such as kittens and seniors. It is
also very contagious from cat to cat(not to humans) so if one has it most
likely others will too. The vet visit I'd say is essential. The vet may give
antibiotics but only as a prevention of secondary bacterial infections (Cat
flue is caused by a virus, and antibiotics can't kill viruses, but they will
kill potential bacteria that will attack body weakened by a virus). Many
cats do survive cat flue with lots of TLC and care which will be described
by a  vet. Isolation is necessary due to contagiousness of Cat flue as well
as possible secondary infections to your cat. Cats may stop eating thou as
they can;t feel the smell of food. Also they could get dehydrated.

Once you cure the cat flu pls make sure you vaccinate your cat in the future
against this horrible disease. (The vet will NOT do it right now however
while she is ill. ) Cat flu can reoccur just like in humans.

I do hope sincerely that your cats don;t have it though, and that its
something simpler. Please check some links on the web re cat flue so you can
see if symptoms coincide with your cats.

http://www.derbycats.org.uk/newsOct03.htm
http://www.cats.org.uk/html/index.php?sect_id=104 ( to download proper
leaflet on cat flue)
http://www.catworld.co.uk/articlecatworld.asp?artid=388&cat=Open%20surgery&pre=
(pls add the equal sign at the end of link)
http://www.fabcats.org/catflu2.html
http://www.petplanet.co.uk/petplanet/health/vaccinationscat.htm
http://kingtonks.com/cat_flu.htm
http://www.dspca.ie/cats.asp

Best wishes
Gee

(...laryngitis)