In rarer cases, a systemic reaction can occur causing symptoms such as fever, malaise, and enlarged lymph nodes. Systemic reactions tend to occur more commonly in people who have had previous exposure to deer fly bites. It's important to note that the severity of the symptoms can vary depending on the individual's sensitivity and the number of bites or stings received. If you are concerned about a specific bite or sting or if symptoms persist or worsen, it is recommended to consult a healthcare professional for proper evaluation and treatment.