[R] subsets

[Den]

Dear R people
Could you please help.

Basically, there are two variables in my data set. Each patient ('id')
may have one or more diseases ('diagnosis'). It looks like 

id	diagnosis
1	ah
2	ah
2	ihd
2	im
3	ah
3	stroke
4	ah
4	ihd
4	angina
5	ihd
..............
Q: How to make three data sets:
	1. Patients with ah and ihd
 	2. Patients with ah but no ihd
	3. Patients with  ihd but no ah?

 If you have any ideas could just guide what should I look for. Is a
subset or aggregate, or loops, or something else??? I am a bit lost. (F1
F1 F1 !!!:)
Thank you