Could be a lot of things, the doctor should see the baby. Fever of 101-102 is ok and could just be a virus, but once it's 103-104 they worry more about it being a bacteria, which is more worrysome but still not necessarily dangerous.
Could also be a mild reaction from a vaccine if the baby had vaccine shots recently.
Fever temperature is the first gauge of seriousness.. a viral infection should rarely get up to 104. Any fever of 104+ needs close monitoring.105 and 106 is not dangerous in of itself, but its a signal that it could be a bacterial infection and the child may need antibiotics as a prophylactic (just in case).
Fevers are good. Do not automatically reach for ibuprofen to bring a fever down. Personally, I use low dose ibuprofen or tylenol mostly at night, just to make the baby/child comfortable enough for sleep.
Our bodies raise the core body temperate to stop virus and bacteria from being able to reproduce and spread. The primary defense which the human body has, to stop the spread of all viral infections is to produce a fever. The fever is not a symptom of disease, but is actually the body’s primary anti-viral immune system. The fever stops the telomeres on the ends of viral RNA from making copies of itself. The telomeres are like a zipper which unzips and separates the new RNA copy within miliseconds, but the telomeres are temperature sensitive and won’t unzip at temperatures above 101F. Thus the high temperature of the fever, stops the flu virus from dividing and spreading.
Many parents fear that fevers will cause brain damage. Brain damage from a fever generally will not occur unless the fever is over 107.6°F (42°C). Many parents also fear that untreated fevers will keep going higher and higher, up to 107°F or even more. Untreated fevers caused by infection will seldom go over 105°F unless the child is overdressed, or trapped in a hot place. The brain's thermostat will stop the fever from climbing above 106°F.
.. I am not a doctor ...