At about the time of the infection from the meteor had caused some of the animals to mutate into Dinosaurs.  The fecal matter from these mutated creatures contains lots of the microbes that are both terrestrial and extra-terrestrial.

The meteor attracted some alien beings.  The alien scientist learning from the experience in their home decided only to experiment on the fecal matter and vegetations.   From the experiments some of the fecal matter mutated and turned into living creatures.  

Some of the creatures turn into vampires – we now call them leeches.  Due to their origin, they are very slimy.

Image via Wikipedia