My Daughter (age 6) Developed A Fever Last Night (101.3).

My daughter (age 6) developed a fever last night (101.3). Her only other complaint was headache. I checked her ears, throat, eyes, etc. Nothing. Also no respiratory symptoms like cough or runny nose. She has not complained of stomach pains or diarrhea. I even checked her neck for pain or swollen lymph nodes. Now my husband (34) and infant son (12 months) both have fevers. Again, the only other symptom is headache. What's possibly going on?