We are into the season of colds and sniffles. You can't help pick up an infection ... and as soon as you have it you'll be sure to pass it on!
Missiologists who have studied again the Gospels and the book of Acts have realised the same principle at work with the Good News. The Good News is told or demonstrated, people catch on and find faith for themselves. As soon as they have, they start passing it on. Epidemiology and gospel mission are related subjects!
The internet and social media have also taught the general public the same concept, with 'gone viral' now in everyday vocabulary. Inadvertently the Church of England benefited when their proposed paid cinema ad was blocked and then went viral on the internet, probably giving free exposure to many many more people!
John's gospel shows the principle straight away in chapter 1 (verse 41). The woman at the well does the same in chapter 4. The spread of Christians out of Jerusalem in Acts 8 was the exact opposite of infection control: when you want to control a disease you quarantine and curtail movement of people (as happened in Sierra Leone with the Ebola virus), but here the people were scattered ... leading to gospel spread wider and wider. The Spirit knows His epidemiology!
So the question for us is do we inoculate, or do we make the gospel 'sneezable', so to speak? Can we see the gospel truly go viral once again?