You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Stray cats, dogs and other pets are not uncommon, and people often feel obliged to take them in to try to find their owners - but this can be a risky business. Here are some guidelines as to what you should and should not do. 

1. First of all you might think the cat or dog that you have found is a "stray" but in fact it could have a caring home just around the corner. ....so make extensive local enquiries first .
2. Stray animals may be carrying disease - which could put your own animals - or even yourself - at risk.  So, take care not to get bitten and do not handle the animal any more than you have to.
3. If you live in a country in which Rabies is endemic - do not approach a "stray" animal. Call the local dog warden (or equivalent) immediately.
4. A genuine stray animal may be frightened and disorientated, and sometimes this manifests as aggression- so be careful not to get bitten
5. You may find a name and address tag on the collar - if you can read it safely you may be able to find the telephone number of the owner
6. Increasingly pets are microchipped  with an electronic identification system which can be read using a hand-held device. Many veterinary practices have such a device - so phone your veterinary practice for advice about this. Your veterinary practice is also a useful resource for other contact numbers and addresses.
7. Local regulations vary regarding responsibility for stray pets. In the UK you may find a local Dog Warden Scheme in operation, in other areas the Police are responsible for stray dogs - but not for stray cats. Also in the UK Animal Charities - eg the Royal Society for the Prevention of Cruelty to Animals, the National Canine Defence League or the Cats Protection League, as well as numerous other Rescue Shelters might have branches in your area which will accept stray animals - ask your Vet, check in your telephone book, or ask Directory Enquiries.
8. Once you have "captured" a stray - keep it confined so that it cannot escape - in a pet basket if it is small enough.
9. Keep the stray isolated from your own animals - in case it is carrying disease
10. Do not feed a stray immediately, but do offer it fresh water to drink.
11. If you have plenty of time and money you might consider placing adverts in local shop windows and in the press. Some local radio stations and local newspapers have lost and found pet sections - and sometimes these are free services.
12. If you can not find the owners and decide to keep the animal (assuming you are legally entitled to keep it), get it checked by your veterinarian, keep it isolated for a few days to see if any signs of disease appear, make sure it has vaccinations and is treated for common diseases (eg worms, fleas). 
13. Check the legislation that applies in your locality. For example, how long after you adopt a "stray" can the original legal owner reclaim it ?