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I work directly in fixed income corporate finance and first of all, the financial markets have not virtually frozen up and there is still plenty of CP and bonds being sold. The main securities being affected are those held by those specifically in the residential sector, primarily mortgage backed securities and other asset backed securities. Ratings for these companies do not at all affect the credibility of other issuers. The impact on fixed income corporate finance has been minimal at best.
It has mainly been caused by the issuance of subprime mortgages (people with bad credit given terrible mortgages with bad terms) and other adjustable rate mortgages. These loans were made to many different people, not illegal aliens. These mortgages did not have fixed interest rates. When you have an ARM (Adjustable Rate Mortgage) your payment goes up after the teaser rate period ends. Needless to say, many people did not understand their mortgages when they signed up for them and when their payments went up, they were unable to make them which lead to foreclosures.
The lesson to learn, you need to deal with lenders and financial advisors who have the heart of a teacher and will take the time to help you understand. Also, we need to educate ourselves so that we can make decisions on our own that we can trust.
Hope this helps!