Problem: The graph shows the number of cases of a disease like tetanus over time after the introduction of a vaccine for it in 1940. The jagged line shows the number of people who got the disease per 100,000 people. The straight line is the trend line, which shows the correlation between the number of cases of the disease over time. Based on the graph, describe the correlation between cases of the disease and years since introduction of the vaccine. Graph with Cases per 100,000 people on the y-axis and Year on the x-axis. Blue trendline goes from 100 in 1950 to zero at year 2000. Red line zig-zags along trend line.