Why Do Some Bug Bites Become Hard or Swollen?

Some people are sensitive to or have an allergy to bug bites. Often it is the saliva or substance that gets injected into the bite from the bug that is the substance that causes the reaction that leads to swelling, redness, pain and hardening (induration). Less commonly, a severe allergic reaction can lead to systemic symptoms like vomiting, full body rashes, throat swelling and loss of consciousness akin to an anaphylactic reaction. Common bug bites that can lead to this reaction include mosquito, fly and spider bites as well as bee stings. Another reason bug bites can lead to swelling, redness and pain is infection. The break in the skin caused by the bite can leave an opening for bacteria to enter leading to infection. Additionally, people may scratch a bite leading to skin breakdown that then leads to an opening for bacteria to cause an infection.