Ever wonder why so many folk songs are about milkmaids and shepherdesses? It's because they never caught smallpox. They were not covered in the scars that marred the beauty of women even in the highest society. Multiply the size of chickenpox scars by three and their number by a hundred and you get the picture of a smallpox survivor.

Cows and sheep carried their own forms of 'pox' that the country girls caught in childhood. Often there was only one spot, usually on a wrist, and a very minor illness, but it left them immune for life against smallpox.

The link between the viruses was made in the 18th century, and it started the science of vaccination. Material from a cowpox scab was inoculated into children to prevent them getting smallpox. Two hundred years later smallpox is extinct.

The experience with smallpox and other vaccines led to predictions 100 years ago that diseases transmitted from animals to humans would be eradicated within the next 50 years. Pasteurising milk dramatically reduced the numbers of cases of tuberculosis transmitted through cows' milk. It virtually eliminated brucellosis, also passed to us in milk from cows and goats, that causes arthritis, muscle aches and pains, and miscarriages.

Pasteurisation almost wiped out listeriosis, also transmitted in milk and cheeses. It causes deaths from pneumonia and brain infections in small children and older adults, and can damage infants in the womb of women drinking or eating infected material. No-one should underestimate the dangers of returning to unpasteurised dairy products - these germs are still around.

Louis Pasteur also tackled rabies. Caught from the saliva of a rabid animal - usually a domestic pet that was in contact with a wild animal - it inevitably led to an agonising death. Pasteur made the first rabies vaccine, new versions of which are still saving millions of lives.

Yet as one disease is conquered, others arise. The 20th century saw an increase, not a fall, in the diseases we know we get from animals. The worst have been human immunodeficiency virus (HIV, that gives us AIDS), and bovine spongiform encephalopathy, that has given us the new variant Creuzfeldt-Jakob disease (CJD).

Sometimes it's difficult to link a disease with animals. For years doctors were puzzled by outbreaks of 'Q fever'. People with it developed pneumonia and liver disease: some recovered quickly, others went on to chronic illness. Now we know it is caused by a bacterium called Coxiella, released from sheep or goats. Coxiella spores can be carried miles on the wind, affecting people never in direct contact with the animals. In Birmingham in the spring of 1989, 147 people developed Q fever. All were in an area 11 miles long by 4 miles wide, directly in the path of winds blowing from sheep farms during the outdoor lambing season. Exactly the same happened in southern France five years ago. Q fever affected 289 people after the wind blew from an area where there were 70,000 sheep.

Most diseases, however, come from household pets. You can catch them from bites, scratches or licks. Never kiss a pet or let it lick your face, or touch the fur of a pet you suspect has skin trouble. 'Cat scratch disease' can make people ill for months: it is caused by a germ called Bartonella in the cat's claws, and treatable by appropriate antibiotics.

Dogs spread toxocara, a parasite in their excrement. Dogs and playgrounds or sports fields don't mix. Last year German vets reported that one in six samples of dog dirt contains toxocara. Children, athletes or footballers contaminated with it run high risks of serious eye problems: toxocara migrates into the eyes, and can cause blindness. British dogs are probably no cleaner than German ones.

By all means have a cat or dog or other pet, but understand that they, and birds, tortoises and even fish can spread diseases. Be sure that they are free from them. Take them for regular vet checks, which include all the vaccinations they need to keep them, and you, disease free.

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.