Therefore要加逗号,后面跟句子（注意是句子）；
Thus不一定要加逗号,后面不一定跟句子.
I was helping my sister with her study.Therefore,I failed to pass the exam.
Air carry the virus to everywhere,thus causing the explosion of the disease.（无逗号,thus 后面不是句子）
Air carry the virus to everywhere.Thus,it causes the explosion of the disease.（逗号,thus 后面是句子）