Their immune systems fought the infections. Now, less than 60 years after penicillin made its debut, we are facing a crisis because of bacteria that are resistant to multiple antibiotics . To deal with this, some physicians are advocating shorter courses of antibiotics for some infections. In England they have tried treating some ear infections without antibiotics. As a result the incidence of mastoiditis has gone up, but since many of the infections don't require treatment, most of the kids get better.
Fever resolves with antibiotics when the infection being treated can be cured by the antibiotic.
Virus infections cannot be treated with antibiotics. Antibiotics have no effect on viruses. Antibiotics kill bacteria. That is all antibiotics do. Upper respiratory infections like colds and flu are caused by viruses.