Allah mentions the Ba'sa' and Darra' that struck the earlier nations to whom He sent Prophets. Ba'sa', refers to the physical sicknesses and ailments that they suffered, while Darra', refers to the poverty and humiliation that they experienced,

لَعَلَّهُمْ يَضَّرَّعُونَ

(so that they might humble themselves) supplicate, humble themselves and invoke Allah, that He might remove the afflictions that they suffered from. This Ayah indicates that Allah sent down severe afflictions to them so that they might invoke Him, but they did not do what He ordered them. Therefore, He changed the affliction into prosperity to test them,

ثُمَّ بَدَّلْنَا مَكَانَ السَّيِّئَةِ الْحَسَنَةَ

(Then We changed the evil for the good,) Therefore, Allah changed the hardship into prosperity, disease and sickness into health and well-being, and poverty into richness in provision, so that they might be thankful to Allah for this, but they did none of that. Allah's statement,

حَتَّى عَفَواْ

(until they `Afaw) refers to increase in numbers, wealth and offspring. Allah said next,

وَّقَالُواْ قَدْ مَسَّ ءَابَاءَنَا الضَّرَّآءُ وَالسَّرَّآءُ فَأَخَذْنَـهُمْ بَغْتَةً وَهُمْ لاَ يَشْعُرُونَ

(. . and they said: "Our fathers were touched with evil and with good." So We seized them all of a sudden while they were unaware.) He tested them with this (afflictions) and that (ease and abundance) so that they may humble themselves and repent to Him. However, they failed both tests, for neither this nor that compelled them to change their ways. They said, "We suffered Ba'sa' and Darra', but prosperity came afterwards, just as like our forefathers in earlier times." "Therefore," they said, "it is a cycle where we sometimes suffer a hardship and at other times, we enjoy a bounty." However, they did not comprehend Allah's wisdom, nor the fact that He is testing them in both cases. To the contrary, the believers are grateful to Allah in good times and practice patience in hard times. In the Sahih, there is a Hadith that says;

«عَجَبًا لِلْمُؤْمِنِ لَا يَقْضِي اللهُ لَهُ قَضَاءً إِلَّا كَانَ خَيْرًا لَهُ، وَإِنْ أَصَابَتْهُ ضَرَّاءُ صَبَرَ فَكَانَ خَيْرًا لَهُ، وَإِنْ أَصَابَتْهُ سَرَّاءُ شَكَرَ فَكَانَ خَيْرًا لَه»

(The matter of the believer is amazing, for nothing that Allah decrees for him, but it is better for him. If a Darra' (harm) strikes him, he is patient, and this is better for him, if he is given Sarra' (prosperity), he thanks (Allah) for it and this is better for him.) The believer, therefore, is aware of the test behind the afflictions whether it may be prosperity or adversity that Allah sends to him, as well as the blessings. Similarly, in another Hadith,

«لَايَزَالُ الْبَلَاءُ بِالْمُؤْمِنِ حَتَّى يَخْرُجَ نَقِيًّا مِنْ ذُنُوبِهِ،وَالْمُنَافِق مِثْله كَمَثَلِ الْحِمَارِ لَا يَدْرِي فِيمَ رَبَطَهُ أَهْلُهُ وَلَا فِيمَ أَرْسَلُوه»

(The believer will continue to be tested by afflictions until he ends up pure from sin. And the parable of the hypocrite is that of a donkey, it does not know why its owners tied it or released it.) Allah said next,

فَأَخَذْنَـهُمْ بَغْتَةً وَهُمْ لاَ يَشْعُرُونَ

(So We seized them all of a sudden while they were unaware.) meaning, We struck them with punishment all of a sudden, while they were unaware. A Hadith describes sudden death,

«مَوْتُ الْفَجْأَةِ رَحْمَةٌ لِلْمُؤْمِنِ وَأَخْذَةُ أَسَفٍ لِلْكَافِر»

(Sudden death is a mercy for the believer, but a sorrowful punishment for the disbeliever.)