A live vaccine, also known as an attenuated vaccine, contains a pathogen that causes mild, if any, symptoms.
In this type of vaccine, the pathogen (virus or bacteria) is weakened or attenuated in the laboratory so that it cannot cause serious disease in the vaccinated individual, but still elicits an immune response. The weakened pathogen is still alive and able to replicate, but it does not cause the severe symptoms associated with the disease.
The immune response to a live vaccine can provide long-lasting immunity, and in some cases, a single dose can confer lifelong protection against the disease. However, live vaccines are not recommended for individuals with weakened immune systems or certain medical conditions, as they may be at risk of developing the disease from the vaccine itself.
Therefore, the correct answer is: a pathogen that causes mild, if any, symptoms.