Your archetypal British cat lovers, we live on the edge of Ascott-under-Wychwood, surrounded by farmland and with cattle shippons only 50 yards away. Farms are a magnet to feral cats and with two female moggies of our own, we anticipated problems.

The first problem did not seem so at the time. Thomas was black with a white chest and we bestowed his soubriquet in honour of the infamous cat in Tom and Jerry. Thomas's best mate was the eponymous Tubby, next door's ginger tom, but when the family moved, Thomas made friends with our two and they existed in peaceful harmony. This was both a surprise and a relief, as one of ours might look like the Dairy Cream cat, but has a mighty fierce alter ego.

Apart from regular incursions through the catflap to dine at our felines' table, and the occasional slight afterwhiff, Thomas was no problem. Initially very wary, he grew to trust us and even allowed himself to be stroked on occasions. While scrawny and dishevelled, he seemed quite happy.

One spring morning, we looked out to see an obviously very sick moggy, his lacklustre coat, sunken, haunted eyes and leg ulcers mute evidence of his poor health. I contacted the local branch of Cats Protection for advice. They lent me a trap cage and asked me to take Thomas to the veterinary hospital in Chipping Norton for assessment and treatment.

Thomas duly took the bait and I carted him off to hospital. Two hours later, they rang to say that he had AIDS and they had put him to sleep to save him from a painful death as his system collapsed. Like human AIDS, there is neither vaccine nor cure. Sad though we were, we were glad to have given him a humane release.

The immediate worry was our cats. Our vet performed blood tests and pronounced them fit. These tests are uncomfortable for a cat, as the sample is taken from a leg and, even five years ago, each test cost £40.

Peace reigned for a while, until Blue silently but clearly announced his presences. We awoke one morning to the pungent aroma of cat urine and spent the next 20 minutes with bleach, washing the front door and various areas on walls and skirtings that he had marked.

Blue was actually more charcoal grey, a large tom with a fine coat, bushy tail and luminous golden eyes. He became a grade-A pain.

Hoovering up our cats' food was no great problem, but time after time he would enter the house and spray and we would arrive home to a nauseating stench and yet another round with the bleach.

Even if the catflap was locked, he would mark the front door and the aroma would permeate our small cottage.

Fortunately he did not attack our cats, but we later discovered that he was often at war with our neighbour's two, resulting in numerous wounds and high vets bills. He even sprayed the front door of the adjoining house and they have no cats.

If we approached, he would run off, but we frequently asked ourselves if he was feral as his condition and size seemed far too good for a wild cat.

The coming of summer multiplied the problems. His visits became more frequent and the higher temperatures exacerbated the smell. In winter, our two worthies snuggle up in the warm and we lock the catflap at night, but in the lighter nights they are off huntin', shootin' n' fishin', so the flap stays open. Something had to be done. I borrowed a trap cage once more and Blue instantly took the bait of smelly sardines (pilchards are better still).

We became very unsure that Blue was indeed a feral. His coat was thick and not matted, there were no obvious scars or bald patches, his ears were unmarked and his eyes clear. Most ferals stink of urine. He did not.

Concerned that we might be interfering with someone's pet, we let him go.

Enquiries around the village soon changed our perspective. A friend with cats who lived further down the street confirmed that he was indeed wild and caused her identical problems. For a long time, he had lived in some tin sheds in the garden of the cottage opposite. The elderly lady who owned the cottage had died some months ago and the developers who bought it flattened the sheds. Thus Blue had to seek pastures new and his visits had increased.

I took him to Chipping Norton for assessment, a chip scan, blood tests and neutering. Generally neutering calms down a male, although there is no guarantee. The aim was to bring Blue home, leave him food and water in our lean-to and see how things went.

Within hours, the hospital called. Blue was HIV positive. Although he did not have full-blown AIDS, he was likely to develop it. More seriously, he was a carrier and a danger to all the domestic cats in the neighbourhood.

If he could have been kept away from the other cats and looked after, he might have lived to a ripe old age, but that was impossible. With the Cats Protection and my agreement, they put the poor animal to sleep.

Humane though this was, I still had terrible pangs of guilt and several ladies shed tears.

We have learned a few lessons. If you have a problem with a cat, the chances are you are not alone.

Others may have better knowledge of the cat's provenance if, as we did, you have doubts that it is feral. Trying to live with the difficulties is no solution, they are only destined to get worse. The Cats Protection is very helpful and treatment for these cats is free. A donation to funds or the hospital's is much appreciated, but not demanded.

Last, but by no means least, the chances of ferals having AIDS seem high. So ignoring the problems only heightens the danger to domestic pets.