Long term regular basis fever. Disease.

Hello, My niece is suffering for fever in a regular basis, she is 18 months old, have some growth problems, not growing as a normal child. She has a heart problem by birth but this is okay for now. By birth, she has everything low compared to normal calculation as someone should have to have in a body(I mean blood related and nutrition related things). She is now suffering for fever in every few weeks, sometimes it happens in every few days. I can't consult with a good doctor in my country for covid problem. I'm from Bangladesh. I'm willing to go abroad with her to get better treatment after the pandemic. In my country, no one is able to determine her problem. Actually, I'm trying to know her disease or problem. Then we could get a treatment easily. I'm worried if she have any special type of disease. Thank you.