Several theories have been proposed for why religions and religious beliefs have evolved, but before now none of them have involved parasites.

Previous theories have suggested that religions help enforce group cooperation. Another suggestion is that religious thoughts and practices are a side-effect of mental abilities that have evolved for other purposes. For example, prayer is a small step from our evolved ability to rehearse what we plan to say to someone who isn't physically with us right now.

Crucially, none of these accounts can readily explain why the diversity of religions varies so much around the world. Brazil, for example, has 159 religions compared with Canada's 15, even though both countries are of comparable size.

Now Corey Fincher and Randy Thornhill have tested the idea that religious diversity is a side-effect of the fragmentation of cultures that tends to occur in the face of increased threat from infectious disease.

Fincher and Thornhill used the World Christian Encyclopedia and the Global Infectious Disease and Epidemiology Network to compare the spread of infections and religions across 219 countries. Their results were clear: in regions with a greater variety of infectious parasites, the diversity of religions also tends to be greater. This association held strong even after exploring the impact of other potential factors, such as differences in democratisation and histories of colonisation.

The researchers say the association between religion and parasites occurs because reducing contact with outsiders can help protect against disease. In turn, when cultures fragment and groups avoid making contact with each other, more religions are likely to spring up.

"Although religion apparently is for establishing a social marker of group alliance and allegiance, at the most fundamental level, it may be for the avoidance and management of infectious disease," Fincher and Thornhill said. The pair also believe that the diversity of languages and parasites tends to co-vary across the globe for similar reasons.

一些探索宗教和宗教信仰的演变原因的理论已经有人提出了，但到现在为止还没有任何相关理论涉及寄生虫。

以前的理论认为，宗教促进人类的团体合作。也有的认为，宗教思想和实践是人类用于其它目的所形成的心智能力的一种副作用。举例来说，祈祷来源于人们排练对不在身旁的人说话的内容。

最重要的是，所有这些理论都不能解释世界上宗教多样性的原因。例如，巴西有159种类型的宗教，而加拿大却只有15种，即使这两个国家的面积大小相关不大。

现在科里‧芬奇和兰蒂‧陶希尔已经证实他们的想法：宗教的多样性是文明分散的一种副效应。而文明分散往往是因为面临来自传染病不断扩大的威胁产生的。

利用世界基督教百科全书、全球传染病和流行病学网络，芬奇和陶希尔在219个国家内对传染病传播与宗教传播进行比较。其结果是明确的：在传染性寄生虫种类繁多的区域，宗教的多样性也往往会更明显。这种联系对之后探讨其他潜在因素有很大的影响，如不同的民主化历程和不同的殖民统治历史。

研究人员表示，宗教和寄生虫之间的关联是因为减少与外界接触可以帮助抵御疾病。反过来，当文明的片段或人类群体避免彼此接触，更多的宗教就有可能涌现。

“虽然宗教表面是为团体同盟或忠诚建立一种社会标志，但根本上，它可能是为了避免和控制传染病， ”芬奇和陶希尔说。两人还认为，语言和寄生虫在世界各地的多样性可能出于同样的原因。