Suppose a test for Lyme disease is positive with probability 0.9 if a 
person has Lyme disease, and with probability 0.05 if they do not have Lyme. 
If a person is bitten by a deer tick, the CDC says the probability of getting 
Lyme is about 0.01. Suppose a person is bitten by a deer tick and takes the test. 
If the test is positive, what is the conditional probability that the person has Lyme? 
Hint: construct a table of 10,000 people who obey these probabilities exactly. 
Alternately, you could make a tree that splits first on Lyme or No Lyme, and then 
on Positive and Negative.