Through play, players understand group immunity and the need to vaccinate.

The girls won the debate on a 2,436-886 vote, thereby declaring them winners of the challenge and earning group immunity.

For the seventh and eighth nominations, House Competitions also have group immunity in the coming nomination put at stake.

Group immunity also is important because flu vaccines are not 100 percent effective, with some studies indicating that immunization prevents illness in about three-fourths of the people exposed.

Day 24: At the Industrial House, Big Brother announced the launch of "Gender Wars," a series of challenges pitting the male housemates against the female housemates, with group immunity at stake.