proved the rule

[unpakwon]

What is the following in red saying in the context?

The effectiveness of the Salk anti-poliomyelitis vaccine program was measured by statistics gathered in all parts of the country. Isolated cases of ineffectiveness were the exceptions that proved the rule. A talk based on one of these exceptions would not, therefore, convince a parent that the Salk vaccine program would not protect his child.

Thank you.

 

[bhaisahab]

What is the following in red saying in the context?

The effectiveness of the Salk anti-poliomyelitis vaccine program was measured by statistics gathered in all parts of the country. Isolated cases of ineffectiveness were the exceptions that proved the rule. A talk based on one of these exceptions would not, therefore, convince a parent that the Salk vaccine program would not protect his child.

Thank you.

Have a look here: Exception that proves the rule - Wikipedia, the free encyclopedia
and here: The exception that proves the rule