What's the most common disease?

There is a terribly debilitating disease - the most common disease known to man.
​Do you know what it is?
Yep, it's Mondayitis /mʌndeɪˈaitis/
"​Mondayitis" is a feeling of weariness, sadness, apathy and general distress that many individuals feel when starting the Monday morning work week. 
Symptoms include not showing up for work or even calling in, usually triggered by over celebrating. Home cures include late night breakfasts, sleeping late and half-hearted pledges of sobriety - "I'm never going to drink again".