The use of the term "index" here is in the sense of the mathematical concept ("A number or other symbol that indicates the location of a variable in an indexed family or set") rather than the publishing usage ("A detailed list, usually arranged alphabetically, of the specific information in a publication").
In this case, it simply means that each disease has a number assigned that indicates how relatively contagious it is (higher numbers are less contagious). It does not refer to a list of diseases and their respective contagion levels.
I believe it is largely left to the GM to set contagion indices for diseases in their game. There is no list that I am aware of, but a few examples are found in canon publications:
- Leprosy = 7 (HM3, Character 9)
- Red Death = 2 (Dead Weight 3)
There are probably others, but these were the first ones I found.