14.4: Disease more generally

The tiniest predators—infection and disease—differ from what are usually thought of as predators in a number of ways, some of which you have seen. First, the disease organism is much smaller than its victim and not independently mobile. It must be carried by wind or water, or induce its host to transfer it in one of numerous ways.

Second, disease does not necessarily kill its victims. Many diseases, in fact, leave their victims largely intact, the better to transmit the pathogen to another host.

And third, after infection, the prey may become forever immune to future infections, both by that pathogen and related ones. This immunity is created by the enormously elaborate “immune system” of vertebrates and other animals— a system recognizing and killing incoming pathogens before they can incubate and do much harm, and as elaborate and complex as the brain and central nervous system.

One of the great discoveries of the last millennium was that the immune system could be primed to recognize a pathogen before it invaded, though what was happening inside the body was not understood until the twentieth century. Vaccination played a central role in the eradication of smallpox. The twentieth century also saw the discovery of antibiotics such as penicillin, which allow doctors to cure disease after an infection has progressed.

These discoveries show us that humans must be considered separately from plants and other animals, for we have developed special powers against disease. We are not passive prey, and do not simply suffer a disease or make behavioral modifications to avoid it. Instead we actively and globally strive to destroy disease, or subdue it. And we extend these efforts to diseases affecting the animals and plants we depend on.

With respect to disease, plants have distinct properties that are in direct contrast with those of animals. Animals, in general, are high-energy organisms— metabolizing rapidly, moving about, and perpetually pumping oxygen throughout the body. Plants are nothing like this. Rather than hearts and rapid fluid flow to distribute food and oxygen and to cleanse waste materials, plants use the passive effects of capillary action and evaporation. This requires the tiniest of veins, or capillary action will fail. And these veins are too small to transport plant cells, or to allow larger pathogens like protozoa and many bacteria to gain access to the entire organism. This also means that plants cannot have the same kind of immune system as animals, with their own cells traveling through their tissues on patrol.

In addition, plants are typically modular. An infected part—leaf, flower, or whole limb—can be discarded and grown again. The apical meristem cells at the tips of branches and roots are capable of developing entirely new plants. While cancer cells can spread through the body of an animal and kill it, such cells would simply plug the veins of plants. So while animals get cancer, plants get cankers. Plants have longevity, while animals have mortality—the cost of being a high-energy organism.