in-context suggestions => sensitive
critical
serious

According to Cancer.Net

Causes of delirium

Delirium often has more than one cause especially for those who are frail or severely ill.

http://www.cancer.net/navigating-cancer-care/side-effects/me...

Causas del delirio

El delirio tiene con frecuencia más de una causa, especialmente para aquellas personas que están en estado delicado o gravemente enfermas.

http://www.cancer.net/es/desplazarse-por-atención-del-cáncer...

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At the top of page 40

Treating frail, older adults
Treatment of AML in people under 60 is fairly standard. It involves cycles of intensive chemo, sometimes along with a stem cell transplant (as discussed above).

http://www.cancer.org/acs/groups/cid/documents/webcontent/00...

At the top of page 45

Tratamiento de adultos en edad más avanzada o en estado delicado
El tratamiento de las personas con AML que tienen menos de 60 años es bastante estándar, y conlleva ciclos de quimioterapia intensiva, algunas veces con un trasplante de células madre (como se discutió anteriormente). Muchos pacientes mayores de 60 años son lo suficientemente saludables como para ser tratados de la misma manera, aunque algunas veces la quimioterapia puede ser menos intensa.

http://www.cancer.org/acs/groups/cid/documents/webcontent/00...

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In the references I've just posted you can also read the following:

People who are much older or are in poor health might not do well with intensive chemo. Treatment of these patients is discussed below in “Treating frail, older adults.”

Las personas que tienen mucha más edad o no tienen buena salud puede que no reaccionen bien a la quimioterapia intensiva. El tratamiento de estos pacientes se discute más adelante en “Tratamiento de adultos en edad más avanzada o en estado delicado”.

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http://www.proz.com/kudoz/english_to_spanish/transport_trans...

guarded:
of uncertain outcome; said of a patient condition.
http://medical-dictionary.thefreedictionary.com/guarded


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Note added at 47 mins (2016-11-09 18:20:34 GMT)
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548,000 results, if you go by numbers.

This is a difficult one to translate as the word is used rather colloquially in Mexico and may encompass a range of meanings. If you want to compare it with standard terms used in the US, I would say this is about the best.
The difficulty is neatly described in the excerpt below from a Trémedica article precisely on this question, and if you don't agree with my appreciation, the article will in any case help you decide.

Incidentally, I've gone with this term based on those mentioned in the Wikipedia entry you cited, as I think it ranks slightly below "serious" but above "fair" on this scale:

Undetermined
Patient awaiting physician and/or assessment.
Good
Vital signs are stable and within normal limits. Patient is conscious and comfortable. Indicators are excellent.
Fair
Vital signs are stable and within normal limits. Patient is conscious, but may be uncomfortable. Indicators are favorable.
Serious
Vital signs may be unstable and not within normal limits. Patient is seriously ill. Indicators are questionable.
Critical
Vital signs are unstable and not within normal limits. Patient may be unconscious. Indicators are unfavorable.
Dead
Vital signs are persistently absent.

Whereas the following scale is slightly more nuanced, and I'd say "serious but stable" would be an appropriate equivalent:

grave, extremely critical, critical but stable, serious but stable, guarded,[2] satisfactory
https://en.wikipedia.org/wiki/Medical_state


delicado
2 Estar alguien delicado Tener alguien la salud dañada y expuesta a empeorar
Luis F. Lara, Diccionario del Español Usual en México


...tanto trabajadoras sociales como familiares suelen preguntar sobre el estado de salud de los pacientes hospitalizados e, invariablemente, la discusión se plantea entre ¿está el paciente delicado o está grave? y ¿tiene un pronóstico reservado? De acuerdo con el DRAE,5 las acepciones que más se acercan al estado delicado de un paciente, serían: ‘2. débil, aco, delgado, enfermizo 3. fácil de deteriorarse’ y ‘5. difícil, expuesto a contingencias’; sin embargo, lo que solemos entender acá es que el paciente tiene un padecimiento que no pone en peligro su vida, o sea, que no está grave.
http://www.medtrad.org/panacea/IndiceGeneral/n31_cartas_Godi...