You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Mexico_City

Context:
The city grew as the population did, coming up against the lake's waters. As the depth of the lake water fluctuated, Mexico City was subject to periodic flooding. A major labor draft, the desagüe, compelled thousands of Indians over the colonial period to work on infrastructure to prevent flooding. Floods were not only an inconvenience but also a health hazard, since during flood periods human waste polluted the city's streets. By draining the area, the mosquito population dropped as did the frequency of the diseases they spread. However, draining the wetlands also changed the habitat for fish and birds and the areas accessible for Indian cultivation close to the capital.

Question:
How many Indians helped build the infrastructure? 

thousands