It certainly shows up in fiction. While it seems that the flu is the most popular purveyors of pandemic in the current rash of realistic apocalypse-in-progress novels, that old standby, the Black Death (Bubonic Plague) still makes its occasional appearance. It was the causal agent in Terry Nation’s novelization of the hit British TV series Survivors, and also more recently (likely via the meme vector of Connie Willis’ Dooms Day Book) Katherine Amt Hanna’s Breakdown. Rats are associated with the spread of the plague, yet neither novel uses this vector. As it turns out, they are correct. Your various rats (or rat fleas to be more exact) are not required vectors.
In addition, the plague is known to infect an amazing variety of both mammals and birds which makes it an outlier in that category, and opens up all sorts of possible alternative vectors.
Having read Ms. Hanna’s (book- it was good, review later), and working on Terry Nations (good so far –review later), I decided to look at more info on the modern versions of the black death. One item to be wary of, is that (oddly) many researchers leave out many of the occurences. You get constant references to Medieval Europe, but they ignore the much earlier Justinian Plague (which we touched on earlier), and the fact that the last wave of the black death was in Marseille in 1720- hardly the Middle Ages.
So I was happy to find a lengthy piece that went into a much broader cross comparison. I have only quoted the very short concluding remarks.
Lars Walløe, CCBI.NIM.ORG, 2008; (27): 59–73
Most (or all) late medieval and early modern “plague” epidemics were caused by [the bacteria] Yersinia pestis (as were the Justinian plagues (ad 542–767) and possibly the plague epidemic in about 1100 bc described in 1 Samuel).
Most (or all) of the historical European plague epidemics did not involve rats as intermediate hosts. The mode of transmission was from human to human via an insect vector.
Pulex irritans [,the human flea,] may have been the most important arthropod vector in Europe prior to the late nineteenth century, but other ectoparasites (other fleas, lice, etc.) could also have been involved.
In China before 1894 and in Hong Kong, in many (but not all) areas of India, and in cities like Bombay, Colombo, Alexandria and Sydney, the most important arthropod vector was Xenopsylla cheopis, [, Oriental Rat Flea,] and the mode of transmission was from Rattus rattus [the Black Rat] (or Rattus norvegicus [Brown or Norwegian Rat]) to man.
Note that the evidence for the Black Death becoming less virulent over time is rather limited. The primary difference may just be that there is effective treatment that limits both the fatalities, and that modern conditions will often limit the insect vectors. As he notes, “the bubonic and septicaemic forms of plague need a vector organism (most likely an insect) to transmit the infection from one mammal to another.” Modern hygiene is helpful.
Note that it can still be very deadly if contracted. This piece takes the more traditional approach (rat fleas spread it) but gives a little more background information
Saylor.Org, June 2011
The modern bubonic plague has a mortality rate of thirty to seventy-five percent and symptoms including fever of 38–41 °C (101–105 °F), headaches, painful aching joints, nausea and vomiting, and a general feeling of malaise. If untreated, of those that contract the bubonic plague, 80% die within eight days.] Pneumonic plague has mortality rate of ninety to ninety-five percent. Symptoms include fever, cough, and blood-tinged sputum. As the disease progresses, sputum becomes free flowing and bright red. Septicemic plague is the least common of the three forms, with a mortality rate close to one hundred percent.
Ouch. Obviously a disease that you don’t want to become resistant to treatment, or find a new nifty vector. I don’t think Flu’s and Bacteria can easily combine their genetic coding, so fortunately (I hope) you wouldn’t expect a Black Death- Flu.