illegal aggrements and collateral transactions.

1.All illegal agreements are void but all void agreements are not necessaarily illega.. Whether this statement is correct. Pl explain

2, An illegal agreement is  not only void as between the immedaite parties it has further effect that even the collaterl transactions to it become tainted with illegality. Collateral transaction is one which is subsidiary, incidental or auxiliary to the original contract. Can any one explain collateral transaction and with example i,e subsidiary, auxiliary and incidental. ?

 thanks in advance