According to Beskid C (From yahoo answers) 

Piric acid's chemical makeup can denature proteins and by altering tertiary configurations of proteins. Piric acid can precipitate and alter H-bonds and van der waals dispersion forces. This changes the hydrophobic end to it's outer stratum and the hydrophilic end binded to the acid. (Think of it as if it flipped inside out :) ) 

This is important because hydrophobic = meaning it repels water ; hydrophilic = it gets water.

The precipitation makes it an insoluble substance. So no pathogens (germs/viruses) can enter nor any important fluid can leave.

I hope this gives you a better understanding on Piric Acid :)