The volunteers in this experiment were treated like the stars of a soft-core version of Saw. They were infected with a cold using nasal drops (75 percent of 159 participants became sick), then isolated in a hotel for five days. Their subjective experience of symptoms and self-reporting of loneliness at large provided the basis for the study. Though, loneliness was only measured once and not against variables like diet or quantity of sleep. While feeling lonely didn't affect the likelihood a participant became sick, it did impact the severity of their symptoms.
It's just one study, but understanding how this cycle can occur -- loneliness making an illness worse, driving further isolation -- can help doctors better understand the patients who live in this merciless world of runny noses, sore throats, and night sweats.