The rise of bird flu across farms in California has raised questions about the virus and its potential threat to humans. While the threat to humans is currently small.

When, where and how the H5N1 bird flu virus may evolve and its capacity to spark a pandemic is hard to predict — in part, some researchers say, because of federal restrictions on gain-of-function research.

Federal authorities say it is likely being carried from farm to farm on objects like farm equipment or clothing that has touched a sick cow.