Final answer:
The statement that a modified live vaccine can produce a fever or mild form of the disease is true. Live vaccines use an attenuated virus that triggers an immune response without causing serious illness, but carry a small risk of back mutations that can revert the virus to a disease-causing form.
Step-by-step explanation:
True, a modified live vaccine can indeed sometimes produce a fever or a mild form of the disease. This occurs because live vaccines utilize a weakened form of the actual virus, which is capable of causing infection but is typically not strong enough to cause the full-blown disease. The process of attenuation includes growing the virus in laboratory conditions that are different from the host environment, such as in tissues or at temperatures not usual for the virus, thereby inducing mutations that weaken the virus. However, there is a low but significant risk of back mutations, which can revert the vaccine strain back to a disease-causing form, as witnessed in Nigeria in 2007 with a polio vaccine outbreak.
By stimulating an active infection, these vaccines aim to activate a robust immune response, which includes both cellular and humoral immunity, and the development of long-lasting memory cells.