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he small brown dog you ordered has arrived,'' said the message on our answering machine that rainy February Sunday afternoon. It was the Dog Lady of Los Angeles. ``No payment is required. You only need to pick her up.''
       Amy and I affectionately call my sister, Karen, the Dog Lady of Los Angeles.  She keeps treats in her car to lure strays and either reunites them with their owners or finds them homes.
   Tug had been left in the women's restroom at Farmer's Market. She was found in a box next to the garbage can.
   That night, we watched a dirty, four-legged creature walk around Karen’s courtyard in that undriven death march of the homeless, less like a puppy than scrawny pink skin held together by exhaustion and despair. We couldn't tell what kind of dog she was.
   Amy shook her head.
   ``I just don't know about this,'' she said.
   ``We'll try it for a week,'' I suggested. ``And see what happens.''
       I was the one who wanted a dog. Our summer wedding was a few months off. We had two cats, but dog helps makes a home.
   But the enemy had already invaded and was assembling microscopic armies against us.
   We didn't know.
        Amy paged me after Tug's first trip to the vet, for shots and a checkup.
   ``I have one word for you,'' she said. ``Mange.''
       She would need weekly dips for the next four weeks to eliminate the microscopic parasites that had eaten her brown and black fur. We had to keep the cats away from her, using spray bottles to drive them off, sometimes having to leave her in the bathroom alone. But classical music soothed her.
   For the first two nights, the death march walking continued. I'd find her in the dark, where she paused on one more circuit around the bed and fell asleep.
   Then, she realized she didn't have to walk, that she had a home. She rested, ate, and slowly the real dog emerged. We bought squeaky toys and gave her oatmeal baths to restore her coat.
   ``You really picked a great dog,'' Amy said one day.
   That morning, after her walk, Tug had sat down to protest the routine being broken. ``It was as if she was saying, `You know you are forgetting something, aren't you?' Amy said. 
       She had forgotten to let her play with the toy in her pocket. So, they played for about 15 minutes, Tug chasing in and around Amy's legs.  ``I love this dog,'' she said.
   As an airborne virus, distemper is usually breathed in from the aerosol nasal discharge of an infected dog, although sometimes it can be ingested, said Dr. Janet Foley, an infectious diseases fellow at the Center for Companion Animal Health at U.C. Davis. It is a highly unstable virus that does not survive more than a few hours outside of a dog's body. The virus quickly spreads in enclosed environments, like shelters.
   Within 24 of exposure_before she was found in that restroom_the virus had already reached Tug's tonsils and lymphatic system. It was attacking her natural defenses first. By the second or third day, the virus was attacking her mononucleaic cells, a class of disease-fighting white blood cells. A fever can develop within four days, Foley said.
      On her second trip to the vet after we had her for a week, Tug was prescribed an antibiotic for a runny nose. At this stage, distemper is indistinguishable from a lesser problem, like kennel cough.
   But it was now attacking the epithelial cells in the lining of the respiratory tract. We didn't know.
   We made the pills into treats, wrapping them in little pieces of bread and made her sit for them. She learned to sit quickly for them, as if to tell us, ``I will sit because I love you.''
       Amy took Tug to work with her in Sherman Oaks. She became a favorite there and at the vet's. Tug followed her around the office. Tug wagged her tail and squirmed when I came to pick them up at the end of the day.
   ``She has the will to live now,'' one of Amy's coworkers said. ``That's the difference.''
       In distemper, the course of attack varies with the individual strain of the virus and the ability of the animal to fight the disease. Puppies, especially those who have not had any natural immunity from mother's milk, are least able to fight it off. Between the third and the ninth day, the virus has become wide spread throughout the dog.
   The virus attacks cells with what is called a cytopathic response, invading a healthy cell, causing them to swell up and die.
   The danger of the mange faded, and after four weeks, the gates and barriers we had used to keep her away from the cats came down. I brought her to Moorpark College, where I was teaching journalism, and let her play with the students.
   It was about 7 o'clock on a Monday morning, rather than soiling the rug as usual, Tug barked to go out for the first time and then squirmed and squealed at my feet, waiting for me to take her outside.
   She was housebroken.
        But the enemy had assembled its forces. That afternoon, the virus struck.
   The distemper had destroyed the white blood cells that filled a lining called the meneges - between the spinal cord and brain - that had been defending Tug's grey matter.
   After the first seizure, Tug was confused and barking. Amy and her boss had to corner her before she could take her to the closest vet she could find, in Studio City.
   What had happened was similar to using a pair of wire cutters to strip the insulation from an electrical wire. Nerve cells are protected by a sheath of cells called mylen. If the mylen decays, ``it is very much like a short circuit,'' Foley said.
   The mylen is not necessarily destroyed by the distemper, but sometimes is destroyed by the dog's own antibodies trying to attack the virus.
   Tug's first seizure was brief. The Studio City vet let Tug go back to the office. Epilepsy was one possibility. Or maybe she ate a spider.      Or there was something called distemper.
   Amy left Tug at the office with her friends so that she could work a shift  at her other job as a bartender. I was planning to pick her up when my pager went off again.
   It was raining.
   Tug had another seizure, much worse, and was back at the vet in Studio City.  The rain was chopping into fine pellets making it difficult to find the right building on Ventura Boulevard. I waited for a half-hour in a little room until I could meet Dr. Stein.
   Distemper was still only one of a handful of possibilities, until they got the blood tests the next day. If it was distemper, well ``Distemper dogs usually don't live,'' Dr. Stein said.