The Black Plague (or Black Death) was a disease that followed the trading ships from China into Europe. It was carried through the fleas on rats and caused a rapid and extremely painful death through swellings (buboes) and vomiting. Any disease that kills a third of the population (around 200 million people in Europe) has devestating affects on the population.
Towns closed their city gates to strangers and trade suffered. The disease lasted from 1328-1351 (1348-1350 in England) and it wiped out whole families, villages and communities. It left a devestated and uncertain population. There was a reactionary wave of religious fundamentalism, which included flagellent groups, but it really led to a breakdown of faith in established Christianity that led to the Reformation in later centuries.
The Black Death also had ramifications in wider society that in England directly linked to the Peasants revolt of 1381. The death of so many meant that labour was in very short supply, feudalism faltered and ended and this meant that farm labourers were able to move from one farm to the next looking for better wages.
The desire of the landed classes to re-establish control led to the Peasants Revolt. The Black Death was awful for those who died, but for the survivors it led to more chances. The Black Death further changed society as it began a movement from the countryside into the towns and also saw a move in farming to pasturing as it was less labour intensive. We can see a further impact in this in the growth of the english woolen industry.
The negative impact of the disease, apart from killing 200 million people was the negative impact it had on peoples attitudes to 'outsiders.' The population turned on minorities such as the Jews who were blamed for the poisoning of wells (not possible in England as the Jews had been expelled pereviously from the country) .