KITCHENER - First Stratford, then Woodstock.

Now three more cities in southwestern Ontario have been hit by cyber-attacks.

A manufacturing company has been targeted in Strathroy, while the online system that serves hospitals in Listowel and Wingham has been hacked.

The hospitals have been forced to temporarily close oncology and diagnostic clinics—those seeking non-emergency care are being told to come back later.

"I don't have a specific timeframe for when we'll have it back up and running," says Karl Ellis, CEO of the Listowel Hospital Alliance.

Hospital officials say the OPP's cybercrimes unit is investigating.

Computer experts say ransomware is a type of malicious software that denies access to a computer system until a ransom is paid.

"Unfortunately we can expect more attacks just like this to target our governments, healthcare institutions, major companies, corporations," explains tech analyst Carmi Levy.

"Just like you plan for a major fire, a flood, a tornado, a hurricane, you also have to plan for a cyber-attack, and you have to have a section there on it."

There's concern because the healthcare system in Ontario is so interconnected online.

Officials in Listowel are doing what they can to prevent the ransomware from spreading.

Earlier this week, the City of Woodstock was hit by ransomware.

Last month, Stratford city officials said they paid out $75,000 in ransom to hackers to unlock their data.