Furthermore, it's often not enough to understand and react to the immediate, proximate causes of disease; it's also important to understand the complex causal web involved in disease transmission.
For example it's strictly true that dirt, filth, and shit in the gutters is not the proximate cause of diseases such cholera: cholera is caused by the Vibrio cholerae bacterium. One could, in one sense, disclaim moral responsibility for permitting or encouraging unsanitary conditions by pointing out the true cause of cholera. "Filth doesn't kill people, bacteria do!" One could also truthfully point out that many people who live in sanitary conditions contract cholera, especially when it's endemic in nearby unsanitary conditions, denying even the indirect causality.
But of course unsanitary conditions provide a vector and a reservoir for many diseases. It's not physically possible to control cholera unless you control the vector and reservoir. Once you understand the scientific connection between sanitation and cholera, it becomes hypocritical to both denounce cholera but not take active steps to improve sanitation; any "moral" arguments to the contrary fall flat.
So it is with religion: Once we understand that religion contributes to death and suffering, we assume a moral duty to find and implement effective measures to break the causal chain of events not at the most
Accepting the disease model of religion entails a number of conclusions.
Religious "liberals" should be viewed as carriers, in just the same way that asymptomatic people can carry diseases such as typhus or tuberculosis. It's absolutely irrelevant that such people themselves do not act in extremist, violent or otherwise aberrant ways: they infect others, whose individual nature renders them more susceptible to symptomatic expression of the disease. Once you accept and propagate the idea that the literary interpretation of ancient mythology is a valid source of moral truth, you contribute to those whose interpretation leads them to embrace violence and extremism. The fact that your personal interpretation isn't violent or extremist is no more relevant than that you personally are asymptomatic when carrying tuberculosis.
The imperialism and colonialism of the West towards the Islamic world has to be seen as promoting and furthering "unsanitary" conditions. Let me be blunt: The proximate cause of Islamic extremism is Islam, and only Islam, not imperialism. But the oppression that results from imperialism and colonialism provides an environment where virulent and extremist strains of the disease of Islam can propagate, just as people shitting in the gutters provides an environment where cholera can propagate. It's not that removing imperialism and colonialism will immediately halt the disease of Islam. It's that we cannot halt the disease of Islam unless we mitigate this environmental factor in its propagation.
Under this model, the Iraq war must be seen as trying to control a cholera epidemic by dumping a million pounds of infected feces where the disease is epidemic, to indicate our "disapproval" of those who live in unsanitary conditions. The motivation is irrelevant; only causal efficacy is relevant.
(The war in Iraq — and especially its supporters' rhetoric — is a perfect example of the specific mechanism by which religion causes dysfunction: it breaks the connection between empirical observation and determination of truth. What becomes important is unfalsifiable assertions about "motivation" rather than empirically determinable causal efficacy. Opponents of the Iraq war are consistently labeled as "pro-Islam", as ridiculous as labeling those who oppose dumping shit into cholera epidemics as "pro-cholera". And religion is not the only cause of this disconnection, as Christopher Hitchens sadly demonstrates.)
One additional conclusion is that there are always unintended consequences to every ameliorative action. For instance, improved sanitation to control infections diseases such as cholera and typhus was indirectly responsible for the polio epidemic of the early 20th century.
Poorer sanitation [prior to the 20th century] resulted in a constant exposure to the virus, which enhanced a natural immunity within the population. ... [Improved sanitation] drastically increased the proportion of children and adults at risk of paralytic polio infection, by reducing childhood exposure and immunity to the disease. [Wikipedia]These unintended and unforeseeable consequences are not an argument for doing nothing, but they are an argument for keeping our eyes open and our minds alert, so we can detect them and mitigate their effects.