A "serious infection" is a strong and harmful invasion of harmful germs or bacteria that can make a person very sick and require special treatment to get better. Full definition
Additionally, the mouth becomes a dangerous source of infection as the bacteria gain access to the blood circulation and cause serious infection of the internal organs. (tri-cityanimalclinic.com)
A retained placenta can be the source of serious infection. (breedingbusiness.com)
Herpes can cause serious infections in newborns because their immune systems are still developing. (babycenter.com)