For a single illness (such as a sore throat and cough), show the demand curves for three different consumers with different preferences about medical care and other goods. Need the demand curves be parallel? Could they ever cross? Now aggregate those three demand curves into a demand curve for them together. If these three people constitute the entire population in a market area, then this demand curve is the “market” demand curve. Think about what would happen when you aggregated the demand curves of thousands of individuals.