What disease is considered a serious disease and where can I find the list in the Tax Guide Book?
The tax relief for medical expenses expended on self, spouse or child of up to a maximum of RM5,000 revised to RM6,000 is available in respect of treatment of serious diseases which is defined to include AIDS, Parkinson’s, cancer, renal failure, leukaemia and other similar diseases such as heart attack, pulmonary hypertension, chronic liver disease, fulminant viral hepatitis, head trauma with neurological deficit, brain tumour or vascular malformation, major burns, major organ transplant and major amputation of limbs.

https://www.thestar.com.my/opinion/letters/...edical-expenses

nota penerangan BE 2019
http://lampiran1.hasil.gov.my/pdf/pdfam/No...an_BE2019_1.pdf

MCO: IRB to shut Hasil Care Line call centre from March 26

https://www.theedgemarkets.com/article/mco-...centre-march-26

KUALA LUMPUR (March 24): The Inland Revenue Board (IRB) announced that its Hasil Care Line (HCL) call centre will be closed from March 26, until the end of the Movement Control Order (MCO) on March 31.

The IRB in a statement on Tuesday said the services offered by the board are also limited to online services such as the filing of the Income Tax Return Form (BNCP) and tax payments through the ezHASiL platform.

Any enquiries can be directed to the board via HASiL Live Chat or by filling up feedback form, available on the website at https://maklumbalaspelanggan.hasil.gov.my/MaklumBalas/ms-my/, Facebook and Twitter.

For further information, taxpayers can access IRB’s official website at http://www.hasil.gov.my

Hadiah wang kepada institusi / organisasi / tabung yang diluluskan

Hadiah wang kepada institusi / organisasi / tabung yang diluluskan oleh Ketua Pengarah Hasil Dalam Negeri.
Subseksyen 44(6) dan proviso, ACP 1967

If not listed, then not eligible for taxable income reduction.
Normally because I do a donation, I check the background of the respective non-profit or charity organization.

Those registered and eligible usually carry a registration number starts with PPM:

For example:

Malaysian Medical Relief Society (Reg. No: PPM-020-14-16091999)

Usually not eligible.
Best to counter check with list in the LHDN database, some may have expired.

If the organisation is not registered, then it’s not eligible for deduction.

Perhaps check with LHDN for eligibility.