Final answer:
An attenuated virus is a weakened form of a virus used in vaccines to prompt an immune response without causing serious illness. However, there is a small risk of back mutations where the virus can regain its virulence.
Step-by-step explanation:
An attenuated virus is a weakened strain of a virus that has been modified so it is less virulent. This is accomplished through methods such as genetic manipulation or long-term culturing in conditions not native to the virus, prompting mutations that reduce its ability to cause disease.
Attenuation allows the virus to still provoke an infection but with diminished strength, which enables the immune system to respond and build immunity without causing significant illness. Despite being more effective than killed vaccines, there is a concern with live vaccines because there's a chance, though low, that the attenuated virus can undergo back mutations. These back mutations could potentially revert the virus back to a virulent form capable of causing disease, as was the case with the polio vaccine mutation in Nigeria in 2007.