'Source-sink' dynamics in polio transmission

• Sources are areas or location where local reproductive success is greater than local mortality (r >0)
• Sinks are areas where individuals are reproducing, but the net reproductive rate is <0 (not replacement)
• Sinks will eventually become extinct if they do not receive immigrants from other areas.

In polio virus transmission it is found that if from one place the virus is eradicated but it may come from other place where polio virus is still circulating, thus maintaining the transmission. For example, Uttar Pradesh and Bihar are acting as 'sources' and the other places in India from where the polio virus transmission has been reduced act as 'sinks'.