Help! Can't figure out what to do!

[Cerebral_Artist]

Every week we recieve a little writing assignment that we turn in during sections, and I can't quite figure out how to answer this weeks problem. Hopefully someone could help.


1. All pathogens require a minimum number of potential hosts to persist but this number turns out to be highly variable among different pathogens. For example, in humans measles requires a minimum of 500,000 hosts but diphtheria can survive with only 50,000 potential hosts.

(a) How should virulence, transmission rate, and latency to infection differ between these two human diseases?

(b) Predict and explain how you think the measles virus would evolve in a long-term lunar space station with a permanent residency of 10,000 individuals.


thanks to anyone that tries to help out.

[Cerebral_Artist]

for part one, I'm guessing the virulency will be worse in measles since it requires a large number of pathogens, but I'm not sure how to explain why. The more pathgoens the more complicated and extreme the disease becomes?

The transmission rate I assume will be higher for diptheria since it requires less pathogens to survive and pass on the disease.

And for the latency to infection, I'm guessing it takes longer for measles since again, it requires more pathgoens. But I'm not sure on how to go about explaining this one either.



And for part b) I'm not exactly sure. Does it spread quicker since there is such a small population, or the opposite? Does it not also depend on how large the space station is? Since the farther away they are from each other, the slower the virus spreads since it's such a small population AND they're far away from other targets.

[woolleyy]

Okay, I don't know if I can answer it all right now, but I think I can give you a good place to start thinking about the question.

As you postulate in your first point, the number of potential hosts needed to persist does increase with virulence, but not for the reason you said above.
What are the least virulent "pathogens" in the human body? The symbiotic bacteria that are present in every person's gut (although they are not strictly pathogens as they do not cause damage to the host, but they work for this example). They require very few potential hosts to persist, as they cause no real damage to the host, and could potentially persist in a single organism for their entire life, which is a long time to allow the "pathogen" to pass on to another host.
The flipside of this coin are the highly virulent lethal pathogens, Yersinia pestis, the cause of "the plague" for example. These pathogens kill the host very quickly so require more potential hosts to ensure that the pathogen is able to pass onto another host before the first host dies.

This also links well to transmission rate, high virulence requires high transmissablility (or the host dies before transmission can occur), and low virulence allows for low transmissability.

Latency of infection is also very interesting, as some pathogens are capable of "hiding" from the host immune system, and bide their time until there are enough organisms to mount a more effective response. This allows more time for transmission to occur, before the virulent part of the infection occurs.

I think I've given you more than enough to think about, and I'll say nothing about part b) apart from that yes, I think that it would depend on the size of the space station, but I would assume an arbitrary size of space station, and I would think more about the topics I have talked about above when answering the question. I would be interested to hear your thoughts on the topics above and part b) also. Hope I haven't bored you...

[Cerebral_Artist]

^ awesome, thank you so much. It makes a lot more sense now!

so for part b, in a small space station, the population would be much closer to one another, making it easier for the infection to be transmitted from person to person. So the transmission rate of measles would be high, leading to the infection of a large portion of the population.
On the other hand, if the space station was large, the population would be more dispersed. This would make it more difficult for the disease to spread from person to person before its original host dies. So the transmission rate of measles would be lower, leading to a small portion of the population getting infected.

[mith]

there's a game on newgrounds.com called pandemic. check it out.

[woolleyy]

Yes, transmission would be high in a tightly packed space station, and lower in a less packed station.

But the question essentially boils down to, would the measles virus survive in a population of 10,000 individuals? The immediate answer is no, because we're told in the first question that it requires 500,000 potential hosts to persist. 10,000 just won't cut it.
The long answer requires you to postulate what pattern the measles epidemic aboard the space station might take. I'll let you figure that out...