In nature there is an abundance of examples of two organisms living together. These can be two plants, two animals, or a plant and an animal. Regardless, the relationship is called symbiotic.
If both organisms benefit from the relationship it is called mutualism. Coral reefs are built by a two organisms that both benefit from their relationship – a coral polyp is an animal that has a plant called zooxanthella living within it. In a commensal relationship such as the mites that live in our eyebrows, one organism benefits without any impact on the other. Humans are the unaffected host for the commensal follicle mite.
The relationship with the greatest “yuck” factor is parasitic, where one organism suffers while the benefits. A broad array of worms, insects, mites, etc plague both plants and animals, sometimes with lethal effects. Complex strategies have evolved over millions of years to allow a parasite to take advantage of the host.
The ability of some parasites to manipulate the behavior of the hosts in nothing short of amazing. The example of the horsehair worm is illustrative. The adult form of this organism, phylum Nematomorpha, is a free living aquatic organism. As the name implies it is long skinny worm. Its life cycle starts as an egg in freshwater. The egg hatches to a larva which lives on the bottom of a lake or stream. Larvae eaten by fish or snails are a dead end, but if consumed by an aquatic insect it can encyst in the body of the insect. When the insect matures, into a mayfly for example, the encysted larva make its way to land. If, after the insect dies, it is eaten by a cricket the hairworm larva has found its destined host.
All this is incidental so far. The real trick is when the larva matures to a juvenile., It has to make its way to water, but crickets don’t normally go to water. Crickets infected with hairworms however change. They seek out water and willingly jump in only to drown, thus releasing the worm back to the aquatic environment. Zombie crickets.
How about zombie ants? The lancet fluke lives in the liver of a cow. To propagate the fluke produces eggs that end up in the animals manure. The eggs get eaten by snails first, then they are passed from the snails to ants – ants like snail slime apparently. An infected ant is then directed by maturing flukes to go find the tallest blade of grass and climb to the top and die. Along comes a cow which grazes the tender tips of the grass. In the process it ingests the dead ant with the flukes, and in so doing completing the life cycle of the fluke.
Even plants and fungi are part of the zombie game. A fungus which inhabits a mustard plant causes the plant to to halt flower production and instead produce small bright yellow leaves that appear to be flowers. These fake flowers contain the reproductive parts for the fungus which are then transmitted to other mustard plants by foraging bees.
Humans have parasites, the only question is, is our behavior being manipulated? Are we zombies?