In a study by Matthew Wilson, which was published in the Journal of Applied Physiology, advanced imaging techniques were used to look at the hearts of three groups of people: 12 lifelong 50- to 67-year-old athletes, 20 age-matched control subjects, and 17 younger (26- to 40-year-old) athletes. Heart-muscle scarring was detected in four of the lifelong endurance athletes and was not found in any of those in the control group nor in the younger athletes. So, the association of heart scarring and endurance running is there. But additional research is needed to draw a conclusion that endurance training causes the scarring and is therefore bad for you.
I would recommend backing off on training when you have a viral illness. A viral infection causes inflammation in your blood vessels, and there is potential to cause scarring in the vessels.
From my personal knowledge, many involved in this research are endurance athletes who have not stopped training. As always, if you have concerns about your heart or if you experience chest pain, unexpected fatigue, shortness of breath, racing heart, or feeling like you are going to black out during or after exercise, you should see your physician.