Warned by both the Regional Specialized Meteorological Center (RMSC) and Joint Typhoon Warning Center (JTWC), Aila formed over a disturbance over the Bay of Bengal on May 23, 2009 and started to intensify and organize reaching sustained wind speeds of 110 kmh (70 mph).

Late on May 21, 2009, the Joint Typhoon Warning Center reported that a Tropical Disturbance had persisted about 950 kilometres (590 mi) to the south of Kolkata, in India and had developed within the Southwest Monsoon.

[1] Environmental analysis indicated that the system was in an area of favorable conditions to develop with low vertical wind shear and warm sea surface temperatures.

[5] In addition, several warning alerts were issued before the cyclone hit Kolkata; however, no alarm bells were rung.

[6] In the Bhola District of Bangladesh, an estimated 500,000 people evacuated to higher areas and shelters as Aila neared landfall.

[7] In India, at least 149 people were killed,[8][9] two by electrocution, and hundreds others were left homeless as torrential rains led to flooding.

[13] The areas and districts affected by the cyclone in West Bengal include East Midnapore, Howrah, Hooghly, Burdwan, South 24 Parganas and Kolkata.

[15] In northern areas of the state, heavy rains triggered numerous landslides in Darjeeling that killed 22 people and left 6 others missing.

High waves produced by the storm inundated coastal villages, forcing residents to evacuate to safer areas.

[22] The remnants of Aila produced gusty winds and heavy rains in the eastern Indian state of Meghalaya between May 25 and 26.

Numerous houses were destroyed by the subsequent flooding and tens of thousands of people were left stranded in the villages.

On the island of Nizum Dwip, nearly all structures were severely damaged or destroyed, leaving roughly 20,000 people homeless.

[37] Several naval relief teams were deployed to the Sunderbans region where an estimated 400,000 people were marooned by flooding.

[39] On 27 May, 400 troops form the National Disaster Response Force were deployed to the state for relief operations.

[43] Five days following the impacts of Aila, the Bangladeshi Health Organization confirmed that a widespread outbreak of diarrhea which has infected over 7,000 people.

[44] Officials feared that the outbreak would lead to many fatalities in isolated areas that have not received aid and have been without food and clean water for nearly a week.

Cyclone Aila has been described as a benchmark of Bangladesh's success in implementation of effective disaster management system.

[46] The joint effort of the Government and NGOs to support the post-disaster recovery was also remarkable but criticized due to lack of comprehensive inclusion of pre-disaster vulnerability reduction measures.