Many infectious diseases can be completely cured—that is, totally eradicated from an individual—even when the parasite responsible has spread throughout the body, but when cancer has spread throughout the body, eradication is a dim hope. People often cite the similarities between cancerous and normal cells as a reason the disease is so difficult to treat, and that is certainly a problem, but I think the ecology of cancer is also fundamentally different from that of invading pathogens.

As Sandy Liebhold brought up in a recent talk, sometimes the ecology of a pest can assist in eradication efforts. Most organisms are subject to Allee effects, meaning that population growth is stunted when only small numbers of organisms are around. Animals that hunt in packs struggle to survive when there are too few organisms to form packs; sexual organisms fail to produce offspring when numbers are so low that finding mates is difficult. These Allee effects help populations on their way to extinction.

Parasites have to contend with Allee effects just like other organisms. Recent work suggests that malaria parasites have to face Allee effects in the form of early, non-specific immune measures. These immune measures appear to be overwhelmed when large numbers of parasites are injected into mice, but small numbers of parasites have a hard time of it. To use drugs to cure a patient of malaria, it is not necessary to kill every last parasite—just to kill enough parasites that immunity can mop up the rest. Cancer cells may not face the same limitations. As cells derived from the host, they are not likely to be vulnerable to non-specific immune measures. Small numbers of cancer cells might even do better, as they face less competition from other cancer cells and may fly under the radar of specific immune measures, which may tend to scale up with numbers. Therefore we may not be able to count on much help from the within-host ecology when we attempt to eradicate cancer from a person.

The situation is not hopeless—sometimes pests spread so far that eradication is deemed impractical. Aside from the economic limitations, it may be that the amount of chemical warfare required to eliminate the pest would decimate the ecosystem just as surely as the pest population would if it were allowed to grow out of control. People instead turn to monitoring the pest populations and treating so as to keep pest damage below a certain level. These ideas form the basis for adaptive therapy of cancer, which attempts to keep metastasized cancer at manageable levels instead of eradicating it altogether. Eradication may not be necessary to maintain individual patient health—managing the problem with a solid understanding of the ecology could be good enough.