DS2 has been poorly for a month off and on with the same virus! It's like flu - fever, aches, headache, sore throat etc.
He has not been this unwell since he had bronchiolitis when he was 5 months old (we were in hospital for a week over Christmas).
Until 6 months ago he was breastfed. Since we stopped he's had a lot more minor illnesses though he also started nursery school in that time. Many illnesses start the day after his 2 days at nursery school! That's fine and to be expected.
Last Friday I was called by my GP to take him to the hospital immediately. They thought he had leukaemia thanks to some worrying white blood cells in the sample he had given the day before (he was unable to walk so I took him to the doctor). Eventually (after the worst weekend of my life) they concluded it wasn't and he got better.
Last night he developed the same symptoms again. This is the third time in a month and previous bursts have lasted a week! I'm exhausted and it's getting us all down.
As a result of these infections he has anaemia which is not caused by low iron. This is under investigation but I believe this can be pretty common in under 5s (is called TEC).
Is there anything safe that I can give him to boost his immunity? When he's ill he doesn't eat much but drinks milk and juice. I'm firmly anti-woo btw 
Thanks everyone - I'm kind of at the end of my tether after a pretty shit month.