XVII.5.1 Just as the way in which parasites are transmitted from host to host affects the level of their virulence, the way memes are transmitted affects how likely and how greatly they will harm their bearer
Some other interesting laws, which have already been described for classical epidemiology, also govern the spreading of memes (Ewald, 1994), see also the chapter XIX.5. Memes (similar to parasites) transmitted exclusively or predominantly vertically, i.e. from parents to children, generally do not harm their bearers, as their successful spreading is closely connected with the fitness of their bearers. In contrast, memes that can spread horizontally can be far more harmful for their bearers. This is especially true of memes that are not transmitted horizontally by direct personal contact between neighbors, but tend to be transmitted over long distances, in the modern world, for example, through the press and television, and are simultaneously not bound to a particular culture, so that they can spread transculturally. Especially harmful memes can spread in populations whose members have a reduced life expectancy for some reason, for example as a result of a war, poor nutrition or diseases. The positive feedback effect can also be important here, where the spreading of the harmful meme reduces the life expectancy of the members of the population, enabling effective spreading of even more harmful memes. The gradual reduction in the occurrence of all possible forms of individual or mass aggression during the second half of the 20th century can be most readily explained as a side effect of the prolonged life expectancy of human beings. The longer this life expectancy, either as a consequence of improved hygiene or advances in medicine, the greater are the penalties for memes that might be successful in the short run, but harm their bearers in the longer term.
The harmfulness of memes is further increased by the possibility of “superinfection”, i.e. increased probability of simultaneous infection of a single person by several memes. If this possibility is negligibly small, an advantage is usually given to those memes that allow their hosts to live as long as possible, so that they are capable of “infecting” a large number of other individuals in the population. However if, during infection by one meme, there is a danger of infection by another meme, even memes that harm their bearers very rapidly can be successful, for example the meme for use of hard drugs. Mutability of memes acts similarly to superinfection. From this point of view, for example, religious systems based on canonized texts will probably be less harmful to their adherents than the religious systems of various sects. Population growth is a factor that can promote the spreading of dangerous memes; memes that are beneficial for their bearers tend to spread in populations with stable sizes or those that are diminishing in size.s