As part of a broader government effort to improve food safety, the Food and Drug Administration (FDA) is spot testing frozen berries because of concerns over hepatitis A virus (HAV), The New Food Economy reports.

In fact, frozen berries have been implicated in four major outbreaks of illness in the United States since the late ’90s. That includes three hepatitis A outbreaks, which caused 405 illnesses and 53 hospitalizations, and one norovirus outbreak that resulted in 136 illnesses. In addition, five years ago, Europe saw a major hepatitis A outbreak linked to frozen berries that sickened nearly 1,500 people.

Experts say each outbreak was likely spread via contaminated water or workers with the virus who didn’t wash their hands properly. Unlike cooking, freezing does not typically kill viruses such as hepatitis A or norovirus on foods, which is a problem because many people eat berries raw.

The FDA’s new sampling program, set to last 18 months, is part of a broader effort by the government agency to use targeted testing to improve food safety across the country. The agency kicked off its efforts with raw milk cheese, whole avocados, sprouts, cucumbers, fresh herbs and guacamole, which are also commonly linked to foodborne outbreaks, before moving on to frozen berries in November.

Moving forward, the FDA will test hundreds of berry samples taken from imports and exports at every point along the U.S. supply chain in order to identify overlooked patterns in the transmission of bacteria––for example, packing methods or regional risks.

To learn more about frozen berries and their history with hepatitis A transmission, click here.