|MadSci Network: General Biology|
This is an interesting question but with a quite complex answer.
Bacterias, viruses and parasites can be quite specific for their host. Some will infect animals without getting them sick while others are able to infect and kill many species. It depends mostly on the type of pathogen studied. To understand how different hosts can react differently to a pathogen, you have to understand how the pathogen interacts with its host. When a blood pathogen gets in the bloodstream, it will usually try to replicate itself very rapidly. If the pathogen is a bacteria, it can do so by itself. If it's a virus or a parasite, it needs some cells or some specific host factors to be able to replicate itself. For a virus (like HIV) or an intracellular parasite (like leishmania for example) to infect a host cell, it needs a very specific receptor on the surface of its host cell. Even if there is probably an equivalent receptor on the surface of the same cell type for many hosts, some small differences between them will lead to a very specific interaction with the virus (human and mouse for example both have a CD4 molecule on the surface of the cells infected by HIV but HIV can only infect the human cells because the human CD4 molecule is a little bit different to the mouse CD4 and it is enough to give the virus its specific infection of humans and not mice). For many pathogens, the host differences between cellular receptors explains why they infect some species and not others or that they kill some species and do not affect others even if they are infected.
Even when a pathogen infects a host without getting it sick, it helps the pathogen by creating a reservoir of infected but healthy animals. This reservoir can then transfer its pathogen to another host via blood-sucking or biting insects (sand flies, tse-tse flies, mosquitoes, fleas, etc) which are then called disease vectors.
Most mammals have the same general components in their blood but each mammal show some small differences between one and the other which explains the specificity of blood (and other) diseases.
The next question could then be "how is it that a pathogen can infect a host without getting it sick?"
The answer then is more a reason of efficient replication of the pathogen. An intracellular pathogen need some very specific cellular factors for its replication and since most factors are a little different between mammals in general, this can explain why a specific disease could be able to infect and replicate but not at its optimal rate. The immune system of the reservoir host could then be able to keep the pathogen in check while in another host it could not. The reservoir host will live and stay healthy while the other one will get sick and could die.
I hope this answered your question. I didn't want to get in too much detail but you can look on the internet about some blood-borne diseases, their cellular receptors and their specific cellular factors.
Here are some web sites you could look at:
http://www.virology.net/cour seware.html http://www.biosci.ohio- state.edu/~parasite/leishmania.html
Hope these will help,
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