1. All infections have their own causative agents.传染病都有自己的病原体.2. All infections are infective.传染病具有传染性.3. Epidemical types vary depending on local geographic and seasonal characteristics.传染病具有地方性和季节性.4. People can be immune to it after having been infected with one epidemic.人们被传染后一般都对该传染病有免疫力.
