Following anti-tourism protests across popular Spanish cities and towns, we are looking for the world's most unwelcome visitor. Our panellists (and producers) are pitching their terrible tourists to see who really is the worst of them all. Some of our contenders include...

The wild boars who's unanticipated vacay to Rome has gone on for so long and caused so much carnage that researchers are putting them on birth control.

The microbes potentially hitching a ride to the moon via space probes and astronauts' poo

The multi-destination parasite who wreaks havoc as it interrails through snails (castrating them on the way), frogs (making them spout multiple limbs) and birds.

But there are some instances when tourists can be a good thing - and this is especially the case in the human body when we want to grow a baby. How is in that we are able to protect what should be an 'unwelcome visitor' from a hyperalert, hostile immune system? Our expert Edward Chuong explains.

Plus, we uncover the DNA origins of the world's most popular coffee bean, hear the freeloading activities of the male angular fish, and read out a selection of your wonderful emails.

Presenter: Caroline Steel