"Study Shows Christianity is Contagious"

I think a big part of it is merely the explosive growth of human population.
The article is extrapolating Christianity itself as 'contagious' because a scientific study named a pattern of virus propagation as Evangelical due to similarities with its spread. Christianity wasn't found to be contagious by the study.

But of course, the fact that the followers of Christ became more and more, eventually converting the Roman Empire, is sufficient evidence of 'contagiousness' of the idea. That and of course the Holy Spirit.
Regardless, CS Lewis also used the concept of a 'Good Infection' to explain Christianity. It is a good analogy, which is why we must continue to spread our Christian virus to a sick world.

Bad theology and demonic forces through willful, witchcraft prayer has expanded "christianity" the last 100 years, but it has not expanded the true Kingdom of God. The charismatic movement has employed new age and occult practices to astonishing effect. The New Apostolic Reformation that has grown from it is the single biggest movement in the world and it seeks to devour all of Christendom in a coordinated effort to draw people away from the simple, true faith in Christ with lying signs and wonders just like Jesus warned us would happen right before His return.

The truth calls those who seek it. Glorious news.