Just hunch - could this be plausible? 
1) Froome had asthma since childhood.
2) Froome hears in 2005 the Bilharzia parasites reduce asthma.
3) Froome deliberately contracts Bilharzia (possible?)
4) Bilharzia blocks his performance on the bike
5) Finally Froome rids his body of Bilharzia
6) then the asthma returns
[before Walsh knew about it]
long shot I know
1) Froome had asthma since childhood.
2) Froome hears in 2005 the Bilharzia parasites reduce asthma.
3) Froome deliberately contracts Bilharzia (possible?)
4) Bilharzia blocks his performance on the bike
5) Finally Froome rids his body of Bilharzia
6) then the asthma returns
long shot I know