1. The basic questions of epidemiology focus on time and place: why here and why now. What are the
answers to these questions for the Broad Street outbreak?
2. Epidemiology relies on non-experimental tests of hypotheses. What was Snow's hypothesis and how
did he test it?
3. How did Whitehead improve on Snow's test of this hypothesis?
4. What is the difference between correlation and causation?

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.