This is wrong.
Have you heard of HIV?
HIV lies dormant in a human body for at LEAST 6 months. Some people live with HIV their whole lives without having AIDS.
You can not detect HIV after you receive it for approximately 6 months, thus disproving your statement.
A lot of food-borne illnesses also stay dormant for 3-4 months.
I can get backing on this information, so if you want it just let me know. ;]