I’m probably being overly anal here, but this something I work with on a professional basis. Preface: when I’m talking about ‘fitness’, I mean the (slightly simplified) biological definition of the term, which means “number of offspring you have before you die”
I think what the OP means is that conditions which allow rapid population growth are conditions which reduce the >natural selection pressure.
This doesn’t need to be true at all—see below.
An extreme version of this is the observation that if everyone survives and breeds, there is no fitness advantage to >any gene and the gene frequencies do not change.
Also not necessarily true (because “everyone survives and breeds” is very different from “everyone survives and has an identical number of offspring).
Selection occurs due to differences in relative fitness, which can be calculated as (personal fitness)/(average fitness of everyone). If everyone has 2 offspring, everyone has a relative fitness of 1. If we have a good year, and everyone has 3 offspring, we have the same relative fitness.
You and OP seem to be thinking about situations in which some sort of environmental limits on fitness have disappeared, and now everyone is limited by some trait for which there is less/no variation. That’s actually a really special situation. Certainly the statement “Some events reduce variation in relative fitness while also increasing average fitness” is true. But so is the statement “Some events increase variation in relative fitness while also increasing average fitness”. Any time conditions increase the fitness of above-average fitness individuals, average population growth increases and selection becomes STRONGER. This is something you would definitely expect in organisms for which individuals actively compete for patchy resources—in a good year, the alpha/owner/whatever of a given territory will get most of the increased value of said patch, and individuals who were without territory may not gain anything at all. (it really depends on the situation, though)
Anyways, average fitness and variation in fitness are two largely independent things. “Rapid population growth”, or even “nobody dies before breeding” doesn’t inherently mean less selection/slower evolution. Sometimes just the opposite (see http://en.wikipedia.org/wiki/Adaptive_radiation)
Typically with the evolution of pathogens, we see a trade-off between the ability of a pathogen to spread (“virulence”) and the ability of the pathogen to keep the host alive (although there’s definitely a lot of variation depending on the life history of the pathogen and the behavior of the host). Overall pathogen fitness (for between-host dynamics—it gets more complicated if the pathogen is competing with other pathogens within the host) is based on (virulence) x (number of other hosts that infected host contacts). So increasing host lifespan and increasing virulence both increase pathogen fitness (but, again, usually increasing one decreases the other). This means that we often see pathogens falling into two syndromes:
a) Fast spreaders (“raiders”) - because they spread rapidly, there is less selection for them to keep their host in good condition (and so it’s better to sacrifice host health to increase spread rate). Alternately, because their host becomes ill rapidly, there is selection for them to be good at spreading. Example would be Ebola.
b) slow spreaders (“farmers”) - because they do not spread rapidly, there is selection for them to keep their host in good condition. Alternately, because their host is in good condition for a long time, there is less selection for them to be good at spreading. Extreme versions of this are pathogens that are largely/entirely transmitted vertically (mothers pass pathogen to offspring). Because the host’s fitness is a part/all of the pathogen’s fitness, there is strong selection for the pathogen to keep the host alive (and even to boost host fitness). A super interesting example of this can be found in the arthropod bacteria http://en.wikipedia.org/wiki/Wolbachia, where some Wolbachia species have evolved into mutualistic relationships with hosts. (But because Wolbachia is only passed from mothers to children, many species change the sex ratio of offspring of infected individuals to be all female. Biology is awesome!)
Decreasing the number of people the host is in contact with is effectively decreasing virulence rate. Because the host (sick person) isn’t going to be in contact with too many potential hosts (other people), there’s a fairly low upper bound on how fit a super virulent/damaging pathogen can be—it’s much more effective for the pathogen to maintain host health as much as possible. Typically we expect this to lead to decreased health impacts on the host. Additionally, this would give hosts a longer time to get access to treatment.
tl;dr Decreasing contact rate is likely to lead to evolution of less virulent/harmful pathogens