First, you might want to learn a little about the Black Death.
Now then. Recently (the last fifty years or so), history scholars have begun to argue that the Black Death was a combination of anthrax and plague, or not even plague at all, just anthrax. They always quote the same two sources:
1. A Florentine doctor's assistant who recorded causes of death in a government ledger; in late 1347, when the plague was at its height in Florence, he began getting sloppy and recording only "the plague marks", which were described once (and only once) as "reddish marks on the skin"
First of all, he was getting sloppy. He was tired of writing down the cause of death because thousands of people were dying and it was this poor guy's responsibility to record why. Second of all, the "plague marks"-- "reddish marks on the the skin"-- could easily describe the effects of the blood vessels hemorrhaging during a case of bubonic plague.
2. An English monk who recorded seeing a field full of hundreds of dead livestock; he said that they were all lying on their backs, feet turned up, and that no one would go within a mile of the field because of the horrible stench.
This would be understandable if it were corroborating evidence-- anthrax is spread by contact with livestock, after all-- but standing alone, it means nothing. If all of the people of a village are dead of the plague, no one is going to feed the livestock. Naturally, they are going to die of neglect, or wander off. As to the strange nature of their position-- this can be explained when compared with other accounts of supernatural animal happenings in Europe during the Black Death. I know its cliche to say it, but people really were superstitious in the Middle Ages (and continue to be so today). For instance, in 1349 a story spread through Germany of massive, man-eating wolves coming down from the mountains and devouring villages. It was wholly believed and spread throughout the country. People make things up to explain phenomena. It's just a fact of historical (and contemporary) accounts.
The final reason that these historians give for their thesis that the Black Death was not entirely or even at all the plague is that no contemporary historian recorded massive deaths of rats and other rodents, which should have accompanied the beginning of a human outbreak of plague (because it is a disease passed to humans when an infected flea's host, typically a rodent, dies and the flea must find a new host). Actually, there is one account of rats dying-- this from one of the best chroniclers of the Black Death, Guy de Chaliac, chief physician to the pope-- but no others, and he makes little of it. Perhaps this is because there aren't that many surviving texts from the period (there are four or five quotes found in every written work about the Black Death, and then maybe one or two others that show up occasionally), but here I have to give the arguing historians credit, because it was a natural phenomenon and it would have normally been recorded. After all, if the University of Paris could blame the pandemic on the astrological position of the planets, and notice all comets and floods in the past hundred years as omens, they should have written about the deaths of so many rodents. Unless, of course, they died in the fields where no one noticed them. Or, by the time anyone did notice them, they were too busy noticing all the dead humans to think about dead animals...
Finally, this theory is slightly ludicrous because of the pandemic nature of the Black Death. Anthrax is not a highly contagious disease, whereas plague is; if one examines a map, and sees the path that the plague took, it will become obvious that it was a disease spread through people, not livestock. It began in Europe with a siege (or with Mongol invaders, depending on what you believe, and they certainly didn't have cows carting around Asia with them) and spread through the ports and the marketplaces. It flourished in the rat-infested cities, especially in the poorer sections, where people lived in close quarters. It did not come from the countryside.
This is, of course, my opinion. Believe what you will.