Q:

makes disaster likely or makes a disaster likely?

A quote from The Economist:

But the central government has the fiscal strength both to absorb
  losses and to stimulate the economy if necessary. That is a luxury few
  emerging economies have ever had. It makes disaster much less likely.

Can we write instead "It makes A disaster much less likely", since disaster may be both a count and a noncount noun? If not, why? 

A:

Yes, you can write the sentence without or without the article. There is a subtle difference that may or may not be implied by the writer and may or may not be inferred by the reader. 
Without the article:

It makes disaster much less likely

disaster refers to a catastrophic set of circumstances that may have developed over a period of time.
With the article:

It makes a disaster much less likely

disaster refers to a single catastrophic occurrence, such as Black Tuesday.

