What the meaning of the following quote?

“you cannot shake hands with a clenched fist”

Read the excerpt from the time traveler's guide to elizabethan england.simon forman, who does attend plague sufferers, is a rare exception: this is because he has himself survived the disease and believes he cannot catch it again. however, his remedy amounts to little more than avoiding eating onions and keeping warm. he has a recipe for getting rid of the plague sores that will afflict you afterward if you survive the disease; but that is a very big "if.” it seems the best advice is provided by nicholas bownd in his book medicines for the plague: "in these dangerous times god must be our only defense.”which lines best summarize the excerpt?