correct word that should be used

And as a nitpick, the gender of your pronouns does not match.

Whereby is correctly used. Most modern speakers of English might not use that word, but it is not incorrectly used, and any reasonably well-educated speaker of English will understand it.

Definitions of illness on the Web:

impairment of normal physiological function affecting part or all of an organism
wordnet.princeton.edu/perl/webwn

Illness (sometimes referred to as ill-health) can be defined as a state of poor health.
en.wikipedia.org/wiki/Illness

epilepsy is a condition, not an illness.
www.epilepsy.org.uk/press/facts.html

Sickness or disease for which no treatment or expense has been incurred for the six (6) month period immediately preceding the effective date of coverage of the insured person whose illness is the basis of the claim.
hartshorn.colostate.edu/index.cfm

An occupational disease is defined as a condition produced in the work environment over a period longer than one workday or shift. Usually an illness is due to repetitive factors over a period of time. ...
www.bls.gov/iif/oshcfdef.htm

A problem in which some part or parts of the body do not function normally, in a way that interferes with a person's life. For the purpose of this module, other terms considered to be synonyms for illness include disease, disorder, condition, and syndrome.
science.education.nih.gov/supplements/nih5/mental/other/glossary.htm

...

http://www.google.com/search?num=100&hl=en&safe=off&rlz=1T4IBMA_en___US219&defl=en&q=define:illness&sa=X&oi=glossary_definition&ct=title

Quote:

Definitions of illness on the Web:

impairment of normal physiological function affecting part or all of an organism
wordnet.princeton.edu/perl/webwn

Illness (sometimes referred to as ill-health) can be defined as a state of poor health.
en.wikipedia.org/wiki/Illness

epilepsy is a condition, not an illness.
www.epilepsy.org.uk/press/facts.html

Sickness or disease for which no treatment or expense has been incurred for the six (6) month period immediately preceding the effective date of coverage of the insured person whose illness is the basis of the claim.
hartshorn.colostate.edu/index.cfm

An occupational disease is defined as a condition produced in the work environment over a period longer than one workday or shift. Usually an illness is due to repetitive factors over a period of time. ...
www.bls.gov/iif/oshcfdef.htm

A problem in which some part or parts of the body do not function normally, in a way that interferes with a person's life. For the purpose of this module, other terms considered to be synonyms for illness include disease, disorder, condition, and syndrome.
science.education.nih.gov/supplements/nih5/mental/other/glossary.htm

...

I'd say either word would work.

Setanta wrote:

Whereby is correctly used. Most modern speakers of English might not use that word, but it is not incorrectly used, and any reasonably well-educated speaker of English will understand it.

He had a rare illness whereby some of her internal organs were not in their normal positions.
What about 'illness'. I believe Noddy is correct in saying that it should be 'conditon': congenital condition

I'd write: "He had a congenital condition in which..."

"Illness" means something has gone wrong.

"Condition" means something was wrong from the get-go.

Drewdad wrote:

I'd say either word would work.

So, to you, it is either 'congenital illness' or 'congenital condition. Did I get your correctly?