A few days ago, Dave Mustaine tweeted the following message:
When a fan pointed out that Mustaine could have Lyme disease, the Megadeth vocalist confirmed "that's what it is."
WebMD described lyme disease as:
Lyme disease is caused by a bacteria, Borrelia burgdorferi, that’s transmitted to humans through a bite from an infected black-legged or deer tick. Symptoms can occur anywhere from 3 to 30 days after the bite, and symptoms can be wide-ranging, depending on the stage of the infection.
According to the Center for Disease Control, early warning signs for lyme disease include "fever, chills, headache, fatigue, muscle and joint pain, and swollen lymph nodes." A rash is definitely visible.
Recovery requires antibiotics and rest.