One episode of Cosmos: A Spacetime Odyssey describes a mechanism by which microbes could travel between planetary systems. As a star's orbit takes it around the galaxy, it occasionally passes through dense interstellar clouds. Rocks thrown into space at that time from the surface of a microbe-harbouring planet could carry life to temporarily nearby planets of another planetary system, before cosmic radiation would sterilise such rocks.
It's an interesting idea for TV; but scientifically, how plausible is this mechanism? My main concern is whether the average number of "infected" planetary systems per passage through an interstellar cloud is estimable as non-negligible.
Note: the use of "panspermia" in the title refers only to the question of whether an occasional interstellar transfer of microbes is tenable, not whether Earth's life originated outside this planet. As I understand it, the mechanism described above is lithopanspermia.