Fortuitous - fortunate
Fortuitous and fortunate are linked in their history, but they have different meanings. Both are adjectives; both have an element to do with 'chance' in their meaning. Sometimes they are confused. This is seen as a malapropism in formal English. Don't confuse them.
- Fortuitous means purely 'by chance'. (It is derived from the Latin forte, used as an adverb ~ 'by chance'. Forte is the ablative ('by') of the noun fors, 'chance'.) It has no connotations of good or bad about it. It is a neutral word, although perhaps more often used of discoveries which people want to remember for their happy outcomes than for the malign discoveries which they would rather forget. There is an adverb fortuitously, with the same meaning.
- fortunate has connotations of good luck, of 'a chance which has benefited' the person we are talking about. We can use the adjective unfortunate or the noun phrase ill-fortune if we want to talk about bad luck. (The Romans had a goddess called Fortuna ('Fortune'), who dispensed luck, and to whom they prayed for good luck.) There are adverbs fortunately and unfortunately. The fact that the Romans prayed to Fortune has resulted in fortunate having much more positive connotations than fortuitous.
The English noun fortune, particularly in the phrase by fortune, is much more neutral than the adjective fortunate. It can be used simply to mean 'as luck would have it'. This is apart from its meaning in terms of personal finance: 'a fortune', in ordinary colloquial speech, means 'a lot of money' - often quite small, in absolute terms: 'I don't know why you smoke - it costs a fortune'. More formally, it means private wealth: 'Andrew Carnegie spent his fortune on charity, notably in founding public libraries'.