Why was Poland spared from the Black Death?

No rats.

Endemic plague only persists in areas infested with rats, which occurred in urbanized areas of Europe, because people were throwing garbage into the streets.

In well-kept countrysides there was no plague, because there was not enough rats.

Poland was an agricultural country centered around manors, not large cities, hence no rats.

Note that to be infected you actually have to be bitten by the flea of the rat, so you literally have to be living with rats to be in danger.

When I traveled Krakow last month, the tour guide explained Black Death affected less in Poland because they had life style sanitizing dishes with vodka.

One factor to consider also is that Poland had a much smaller population than western Europe. Around the time of the Black Death, the Polish population was something like 2-3 million, while the French population was about 14 millon or even higher. It's common sense that disease spreads more easily in higher population density areas, especially when hygiene is poor, as it was in the Middle Ages.

It seems that there is a correlation between exposure to and surviving the plague and a genetic predisposition against infection with HIV that has a prevalence in Northern Europe that is not observed in Southern Europe:

http://www.pbs.org/wgbh/evolution/library/10/4/l_104_05.html

ANOTHER EXPLANATION REGARDING POLAND IS THAT IT HAD A LOT OF JEWS, PARTICULARLY IN URBAN SETTINGS; NOW, WHY WERE Jews Less Affected?

And even if Jews died at a lesser rate, it can be attributed to the sanitary practices Jewish law.

For instance, Jewish law compels one to wash his or her hands many times throughout the day. In the general medieval world a person could go half his or her life without ever washing his hands. According to Jewish law, one could not eat food without washing one’s hands, leaving the bathroom and after any sort of intimate human contact. At least once a week, a Jew bathed for the Sabbath. Furthermore, Jewish law prevents the Jew from reciting blessings and saying prayers by an open pit at latrines and at places with a foul odor. The sanitary conditions in the Jewish neighborhood, primitive as it may be by today’s standards, was always far superior to the general sanitary conditions.

Jewish law also prescribes certain sanitary conditions related to burial of the dead. Leaving corpses unburied not only abetted the conditions that spread the bubonic plague but typhus and other diseases as well. The Jews, on the other hand, had a unique sense of community that not only led them to feel a responsibility to attend to the sick and dying, but caused them to always maintain a formal burial society (chevrah kadisha), whose responsibility it was to make sure that any Jew who died was treated according to Jewish law, including washing the body before it was buried.

These are only a few examples how Jewish law preserved the Jewish people through this terrible dark period of plague. It imposed a sanitary standard on the Jew far above the ordinary sanitary standard that medieval Europe had

Source: https://www.jewishhistory.org/the-black-death/

There are three types of plague, Pneumonic, Bubonic, and Septicemic all of which are caused by the bacterium Yersinia pestis. People infected by fleas get the bubonic form of the plague. However, if the bacteria reaches the lungs, it becomes pneumonic plague which is more virulent spreading via person to person by coughing then no rats are needed since the bacteria becomes airborne

Quite a lot of scientists now think that the plague was in fact airborne and not spread by rats, but by infected people with the Pneumonic form of the plague. This version spreads much faster and kills quicker. Thankfully antibiotics can today prevent the disease from becoming pneumonic (air-born)

Yersinia pestis can survive for at least 24 days in contaminated soil Up to 5 days on other materials link

The travel times and relative isolation were probably enough to stop most of the spread (explained in comments below) ... Considering peasants were not allowed to travel in those days, most likely the plague was spread by traders, this is why the more dense populations were more affected.

With more remote populations kept safe especially since it could kill within 24 hrs of catching it probably those infected died before reaching their destination.

So to help get an idea of travel times: how the distances saved the population I’ve used this map - old travel routes of Europe http://upload.wikimedia.org/wikipedia/commons/e/e1/Late_Medieval_Trade_Routes.jpg so for instance Prague to Krakow (240 miles on google maps) on horse would be 8 days travel (assuming that the roads are flat and that the modern road follows the same path of the medieval trade route. With a wagon pulled by horse 15 - 25 miles/day takes 9.6 - 16 days. With a wagon pulled by oxen 10 - 12 miles/day takes 24 - 20 days (most people who were infected with the plague died within 24-72 hrs so it becomes easy to see that most would die on the way before reaching the more dense populations

I grabbed the approx. horses and wagon travel times here: http://www.terryburns.net/How_fast_could_they_travel.htm

As an aside I think most likely Milan had such a low mortality rate less than 15% because they understood it was people passing the plague to each other. They walled up not only the infected family but the houses on either side leaving them to die. Rats are really great climbers and would have escaped being walled up, just watch these little guys go! :P https://www.youtube.com/watch?v=7o4LrfnX9QQ

climbing up brick (the camera has trouble to keep up!) https://www.youtube.com/watch?v=8bt9Ukw1iB0

So if it had of been the rats spreading the plague walling would have made no difference at all, since they would have simply gone off to the next area and start infecting people again.