Final answer:
Environmental factors such as viral infections that predispose schizophrenia are best illustrated by epigenetic factors.
Step-by-step explanation:
The impact of environmental factors such as viral infections on specific genes that predispose schizophrenia is best illustrated by epigenetic factors. Epigenetics refers to the changes in gene expression that occur during an individual's lifetime. These changes can be influenced by environmental exposures, such as viral infections, and can have lasting effects on an individual's genetic predispositions.