madeinspain- the absence of any cancion-corrido etc. about the 1918 pandemic and others in the monumental 5-volume Cancionero Folklorico de Mexico indicates that such songs are rare- not a popular subject for song. Like Joe Offer, I have a number of Spanish collections and also came up zilch.
I wonder if someone in a comprehensive medical library or doing viral research might be able to help. Doctors sometimes compose poems on such subjects and mail them around but I don't know anyone in those fields.

The National Library of Medicine and the National Institute of Health has placed several recent research papers on-line.
Quoting from one paper on the website, "Phylogenetic analyses suggest that the 1918 virus HA gene, although more closely related to avian strains than any other mammalian sequence, is mammalian and may have been adapting in humans before 1918."
From a linked article: "Sequence and phylogenetic analyses of the complete 1918 haemagglutinin (HA) and neuraminidase (NA) genes show them to be the most avian-like of mammalian sequences and support the hypothesis that the pandemic virus contained surface protein-encoding genes derived from an avian influenza strain and that the 1918 virus is very similar to the common ancestor of human and classical swine H1N1 influenza strains."

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9990079