@High Eight got me reminded of the story "High Eight" (go figure
) by Keith Roberts. It has been a long time since i read it, and this original question got bookmarked with other questions on a search for "Sentient Electricity".
So, one of the best things about question boards is getting a lot of related stories that might or might not answer the question, but will rule stories out, and get a searcher on the path to other boards on similar topics, that eventually find the story.
Here in one place are related stories about sentient electricity and sentient weather. The only part I
haven't found is an electromagnetically spread disease. (I realize that electromagnetism doesn't have to be sentient to do that.) But thanks for the reminder on "High Eight", about electrical power that does spread suicides.
An "SF Magazines" review on "High Eight" is
here. (Writing brief link descriptions in case that particular link fails.)
"The Last Salamander" by John Rackham (not the famous pirate!) is a story about an alien electrical life form that is living in the power stations and lines is available to read online
here.
"The Man Who Made Friends With Electricity" by Fritz Leiber is available to read online
here.
"Nimbus" by Peter Watts is a story about sentient weather and is available to read online
here.
"Press Enter" by John Varley about sentient (and malevolent -- of course!) electrical energy is available to read online
here.
So the only things I haven't found so far is specifically some electrical energy (sentient or not -- was it?) that spreads plagues and pandemics. And a story about electricity that calls it "ghost fluid". Casting a wide net. And catching a lot of interesting stories in it so far.