Can you? I was helping out with a class this last semester and we had this question on the last exam - most people had the right answer, but I completely LOVED these attempts!
"Immunization of large farm animals susceptible to viral infection by a developed vaccine proved to be effective against the virus."
"After initial exposure to an antigen, a subsequent infection with cause memory B & T cells to undergo clonal expansion. A herd of lymphocytes."
"Herd of lymphocytes - large amounts of lymphocytes that provide immunity. Attack antigen."
A correct answer would have to include something about h0w vaccination of the majority of the population provides protection to those that remain unvaccinated - simply because there are less people that can get sick and propagate the disease. Therefore, the "herd" is generally immune. I could get into the many many reasons to get your kids vaccinated, but I figure the majority of the people that read this already know them!
Also, as I was googling "herd" for pictures, I came across this lovely animation that gives a good description of herd immunity - check it out!