.IDX

• One such extension is the .IDX extension. It is attached to a vast array of files, from GIS (geographic information system, or mapmaking software) to Internet services.
  
  

Index

• The .IDX extension denotes an index. This, quite simply, indicates the index of a program, according to filext.com.
  
  

Programs

• According to file-extensions.org, there are a number of programs that use the .IDX extension. These include, but are not limited to, Microsoft Outlook, Quicken and Java.
  
  

Other Programs

• Filext.com cites several other programs that use indexes, namely AOL, but also Corel and ArcView.
  
  

Storage

• Indexes are automatically connected to individual software. It is not a file that needs to be created or altered at any point. In addition, they are not prone to viruses, but a simple virus scan by a software like Norton Anti-Virus should fix any associated problems (though they are rare).