What is an externality? Please give an example of the negative externality. For the flu shot, how does the positive externality shift the marginal benefit curve (the demand curve)? We know that there...

What is an externality? Please give an example of the negative externality. For the flu shot, how does the positive externality shift the marginal benefit curve (the demand curve)? We know that there will be a deadweight loss if consumers purchase the underlying good, which generates positive externality at the market price. Is there a deadweight loss in this case? Why or Why not?