Why should those investigating a crime scene minimize contamination and what are some ways that contamination is limited at a crime scene

Hello!

Investigators at a crime scene must always minimize contamination in order to preserve evidence. If investigators contaminate the crime scene, the evidence could be destroyed or altered. 
Some basic ways that contamination is limited is by controlling the amount of people who can view the crime scene, by wearing gloves, and by avoiding the unnecessary touching and moving of evidence. 

I hope this helps you! Have a great day!