Along with life, liberty, and the pursuit of happiness, there is one other right we like to think we have. That is the right to think for ourselves and make up our own minds.

I wonder if this is true. Humans provide a home for hundreds of life forms that are nothing like us, everything from single-celled bacteria helping our digestion to tiny mites in our eyelashes to the energy-producing mitochondria in each of our cells. All these organisms have their own life needs and biological priorities. Is it possible that we - the conscious thinking humans who believe that we are in charge of all our actions - are controlled at least in part by our fellow-traveling organisms?

There is evidence to support this idea. We can draw some horrifying examples from other species. For example, ants spend much of their lives trying to avoid being eaten. The last thing that an ant intent on survival should do is wait out in the open, motionless and exposed to danger. However, sometimes an ant is invaded by an even smaller animal called a lancet fluke (officially, the species is Dicrocoelium dendriticum). The lancet fluke is an organism that must spend part of its life cycle in another kind of animal, such as a sheep. When the right time comes, the unfortunate infected ant crawls up to the tip of a blade of grass. It hangs on there, in full view, patiently waiting for a sheep to come along and gulp it down with the grass.

Well, you say, that's tough on the ant, but humans and ants don't have much in common. So here is a different example. A second form of fluke, with the just-as-unpronounceable name of Euhaplorchis californiensis, spends part of its life inside a fish and part inside a bird. When it reaches the stage where it is ready to switch from one to the other, guess what happens? The infected fish can't keep still. It jumps and wiggles and wriggles. Any wading bird looking for a meal is likely to notice it and swallow it down, enabling the parasite to proceed to the next stage of its life cycle. The fish doesn't behave like this because it wants to; it does so because it can't help itself.

That is hard luck on the fish, but again you may feel that fish and humans are so far apart biologically that we have nothing to worry about.

So let's come closer to home, with a little one-celled animal called Toxoplasma gondii. Pregnant mothers are likely to have heard of it, at least indirectly, because the creature is carried as a parasite by many cats and also by many humans. A lot of adults, maybe as many as fifty percent of us, carry a passive form of the Toxoplasma organism in our bodies, where it simply seems to lie dormant and causes few or no symptoms. It can, however, invade the central nervous system of developing embryos, when the disease it causes, known as toxoplasmosis, has very serious effects on the developing child. Since the Toxoplasma organism lives in a cat's bowels, and the dormant form of it is excreted from there, any good physician will warn an expectant mother: If you have cats, keep away from the litter box. For the sake of your baby, let someone else clean it out.

Toxoplasma can be bad news for a pregnant woman, but what does it do to a person who is not pregnant? The standard answer, until very recently, was nothing. Even though the passive form tends to be carried as tiny cysts within the brain, these are harmless and have no effects.

Now we are having to rethink that assumption. Toxoplasma gondii can affect rats as well as cats and humans. When rats are infected, an extraordinary thing happens. The rats lose their fear of cats. An uninfected rat will avoid anything that carries a cat smell. The infected rats have no such reaction - in fact, there is evidence that they are attracted to cat smell, apparently forgetting that to a cat, a rat is an acceptable and tasty dinner.

Well, you may say, that's bad news for the rat but it's no big deal for us, because cats don't eat people. On the other hand, in order for the Toxoplasma parasites to force a behavior change on an infected rat, the rat's brain activity must be affected. Is it possible that the brains of people infected with Toxoplasma are also affected, in more subtle ways?

The jury is still out on this one. When a group of people infected with Toxoplasma was compared with a group of uninfected people, there seemed to be no difference in their general health. However, there was a small but significant difference in psychological behavior. Women with Toxoplasma tended to be more warmhearted and kind in their behavior than uninfected women.

This is a single study, and it could certainly be a statistical accident that other studies will fail to corroborate. However, what is undeniable is that cats are around us in large numbers as pets, despite the known danger to unborn children. With all our concern about the unknown effects of environmental hazards such as pollutants in water and in the air, isn't it a little illogical to keep a known hazard as a pet?

If you have a cat, take a good look as it lies there sleeping, or stands waiting for its dinner. Does it seem to have a certain knowing smile on its face?

I don't have cats, but I do have a dog. I also have a T-shirt, which reads, AGENDA FOR DAY: LET DOG OUT. LET DOG IN. LET DOG OUT. LET DOG IN.... It's a pretty accurate description of my daily activities, along with dog feeding and dog walking, and so far I have never complained. But now I wonder just who is setting the agenda.

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