Pharmacies across the country reported difficulties with getting prescriptions following a cyber security issue with the health insurance company UnitedHealth.

Experts say municipal water and other systems have the potential to be easy targets for hackers because local governments’ computer infrastructure tends to be underfunded. Robert M. Lee, CEO of Dragos Security, and a specialist in industrial control system vulnerabilities, said remote access to industrial control systems such as those running water treatment plants has become increasingly common. AdIn May, Israel’s cyber chief s aid the country had thwarted a major cyber attack a month earlier against its water systems, an assault widely attributed to its archenemy Iran. But at about 1:30 p.m., someone accessed it again, took control of the mouse, directed it to the software that controls water treatment and increased the amount of sodium hydroxide. AdRussian state-backed hackers have in recent years penetrated some U.S. industrial control systems, including the power grid and manufacturing plants while Iranian hackers were caught seizing control of a suburban New York dam in 2013.