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
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