Answer:
Hey! Haven't read much of this book before, but hope this helps!
Step-by-step explanation
The most contagious and destructive disease to plague the human race is greed.
"There is a sufficiency for a man's need but not for man's greed." Mahatma Gandhi.
"Selfishness and greed, individual or national, cause most of our troubles." Harry Truman.
For your thesis statement, I would highly suggest going out of the box. The traditional "I believe ....." can become redundant and quite awkward in some essays, so I would recommend maybe starting with "In the book Julius Caesar, ______ are the themes portrayed." Feel free to add on or use syonyms, but hope this helps!