Most viruses need hosts to survive, so why do (the lethal ones) kill their hosts? I know it sounds absurd, but you'd expect viruses to prolong life, not shorten it. So why do they do it?
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Viruses don't necessarily gain from killing people. In some ways, it could be considered that viruses that kill their host quickly are inferior in some ways - if a virus kills its host before the host can spread the virus to another host, the virus will not have any advantage at all! Overly virulent strains could be said to suffer from "short-sighted" evolution (but let's remember, evolution does not have a particular goal in mind).
Virulence in pathogens (viruses, harmful bacteria, and other infectious organisms) is often highly linked to the method of transmission between hosts. This is a result of selection acting at different levels. As soon as a pathogen enters a host, it will begin to replicate. Specific individuals within the pathogen population that reproduce quickly (ie, which harm the host the most) will be selected for by virtue of their quick reproductive rate. They will out-compete their slower-replicating neighbours. But this can go too far - a pathogen that kills its host too quickly risks a reproductive dead-end. If a host can't spread the pathogen to other new hosts, the pathogen will not gain in any way. A balance must be struck.
Here's where the method of transmission comes into play. If a pathogen is spread primarily via direct contact between hosts, selection will favour pathogens with a more moderate virulence. A host that is too sick to come into contact with other potential hosts will doom their carried pathogens by preventing their spread. From the other perspective, pathogens that can survive the external environment for longer periods of time, and be transmitted via water contamination or other indirect means can afford to be particularly nasty. Even if their host dies, the virus will be able to spread through other means. Selection will favour pathogens that reproduce quickly (that harm the host the worst) in this case.
Virulence in pathogens (viruses, harmful bacteria, and other infectious organisms) is often highly linked to the method of transmission between hosts. This is a result of selection acting at different levels. As soon as a pathogen enters a host, it will begin to replicate. Specific individuals within the pathogen population that reproduce quickly (ie, which harm the host the most) will be selected for by virtue of their quick reproductive rate. They will out-compete their slower-replicating neighbours. But this can go too far - a pathogen that kills its host too quickly risks a reproductive dead-end. If a host can't spread the pathogen to other new hosts, the pathogen will not gain in any way. A balance must be struck.
Here's where the method of transmission comes into play. If a pathogen is spread primarily via direct contact between hosts, selection will favour pathogens with a more moderate virulence. A host that is too sick to come into contact with other potential hosts will doom their carried pathogens by preventing their spread. From the other perspective, pathogens that can survive the external environment for longer periods of time, and be transmitted via water contamination or other indirect means can afford to be particularly nasty. Even if their host dies, the virus will be able to spread through other means. Selection will favour pathogens that reproduce quickly (that harm the host the worst) in this case.