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While for example a wolf pack provides protection to a sick wolf, increasing its chance of survival, there is a risk of infecting other members of the pack, decreasing their total chance of survival.

Do animals have mechanisms for preventing that? For example the sick member leaving its group temporarily?

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Diversity in immune genes within the population could be something. – AlexDeLarge Jul 6 at 16:24

Almberg ES, Mech LD, Smith DW, Sheldon JW, Crabtree RL (2009) A Serological Survey of Infectious Disease in Yellowstone National Park’s Canid Community. PLoS ONE 4(9): e7042. doi:10.1371/journal.pone.0007042

The authors found that the majority of Yellowstone wolves had been exposed to a number of different viruses (evidence from antibodies in blood samples).

They also found evidence for historical outbreaks of canine distemper virus in wolves, coyotes and red foxes in 1999 and 2005. These outbreaks correlated with peaks in wolf pup mortality.

This evidence suggests that wolves have no mechanisms for avoiding the spread of these viral diseases. Perhaps periodic episodes of increased mortality are the price that has to be paid for a longer-term 'herd immunity'?

Edit - in response to OP comment below:

The data indicate very high levels of seropositivity to the viruses. I would say that 'not 100% successful' is an understatement. But I must admit that I am way out of my area of expertise here!

From the abstract:

We found high, constant exposure to canine parvovirus (wolf seroprevalence: 100%; coyote: 94%), canine adenovirus-1 (wolf pups [0.5-0.9 yr]: 91%, adults [>or=1 yr]: 96%; coyote juveniles [0.5-1.5 yrs]: 18%, adults [>or=1.6 yrs]: 83%), and canine herpesvirus (wolf: 87%; coyote juveniles: 23%, young adults [1.6-4.9 yrs]: 51%, old adults [>or=5 yrs]: 87%) suggesting that these pathogens were enzootic within YNP wolves and coyotes.

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That's very interesting. But I think the conclusion is not completely correct. I'd rather conclude that "based on the evidence, if wolves have mechanisms for avoiding the spread of those diseases, they're not 100% successful." It could be that they have such mechanisms, and without them, the mortality would be even higher. The last sentence is an intriguing point. Perhaps from evolutionary perspective it's better to let the whole pack/herd become infected and if it survives, all the survivors will be immune. – Petr Pudlák Dec 3 '12 at 10:10

(This answer is still a work in progress, but I'll work more on it eventually...)

It's not terribly controversial to say that infected hosts may be more likely to be eaten by predators. However, at least one author (Smith Trail et al 1980; full reference below) has suggested that this could be adaptive; in some sitautions there could be a net gain to inclusive fitness if, by 'submitting to' predation, infected hosts are able to reduce the force of infection acting upon close relatives.

(Full reference: Smith Trail DR. 1980 Behavioral interactions between parasites and hosts: host suicide and the evolution of complex life cycles. American Naturalist 116, 77-91)

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