Children with allergies to vaccines, and individuals with weak or compromised immune systems, rely on “herd immunity” for protection from disease. “Statistically, there are significant increases in vaccine-preventable diseases during modern times when the numbers of vaccinations in certain areas dropped,” says Shikha Jain, M.D., a hematologist and oncologist at Chicago’s Northwestern Memorial Hospital. In other words, when everyone else is vaccinated, it provides “indirect protection to those who are unable to receive the vaccine,” she says.