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Protests and strikes by fast-food workers are expected to spread to 100 U.S. cities Thursday. This comes a little more than a year after a strike in New York, when 200 workers walked off the job. The new round of strikes will affect McDonald’s Corp. (NYSE: MCD) and other major national fast-food companies.

Workers and their supporters are fighting for a hike of the minimum wage to $15 per hour and the right to form a union without interference. They feel that low wages are not only hurting families and communities across the country, but also slowing the economic recovery and exacerbating the problems of income inequality.

It used to be that an annual minimum-wage income — adjusted for inflation — was sufficient to keep a family of two above the federal poverty line. But that has not been the case since the 1980s. If the federal minimum wage were raised to $10.10 per hour, as some in Congress have proposed, it would bring a minimum-wage income back above the poverty line for a family of three.

Efforts to raise the minimum wage seem to be escalating in the streets as well as Congress. Following the initial walk-off in New York last November, fast-food worker strikes spread to seven cities in the spring and then 60 cities back in August. In addition, thousands of Wal-Mart Stores Inc. (NYSE: WMT) workers protested at some 1,500 stores nationwide on Black Friday.

We asked earlier on Thursday whether McDonald’s and Walmart could afford to pay a $15 an hour minimum wage. Supporters of the current wage structure see increased labor costs decimating company profits. By some estimates, McDonald’s, profits would be cut in half and Walmart’s profit would be reduced by 80%. They feel the interests of shareholders have a place in the debate as well.

It is worth noting that Walmart and McDonald’s topped our list of the U.S. companies paying workers the least, and Yum! Brands Inc. (NYSE: YUM), operator of the KFC, Pizza Hut and Taco Bell chains, was also in the top five.