The answer to this is a bit complicated. Workers' comp does not cover endemic disease, such as common colds or flu, even if they're spread through the workplace. But in general, whether illnesses or pandemics are covered by workers' comp depends on the specific state.
There are endorsements available for work-related travel, in case an employee gets an endemic disease in a foreign country. Also, if the nature of the work being performed makes workers more susceptible to getting a disease, even during a pandemic, then employees might be covered.
More than likely, an investigation will be performed after filing a workers' comp claim for a disease. An employee might be covered for catching an illness during a pandemic if their job's nature specifically put them at higher risk of contracting it. Ask your independent insurance agent for more details.