The expression "lethal illness" is correct and usable in written English.
You would typically use it to refer to an illness that has a high mortality rate or is fatal. For example, "COVID-19 is a highly contagious and often lethal illness."
Anorexia is a lethal illness.
Sle, a rare but potentially lethal illness, can cause inflammation of the brain.
He writes of his own frontier crossing: from health to lethal illness.
By those measures, Karen Pinoci's short but lethal illness shouldn't have rocked me so.
When exposed to the skin, it can lead to Green Tobacco Sickness, a toxic and potentially lethal illness.
Also, he seemed to be hanging on an awfully long time for a man with a lethal illness.
But the scientists were looking specifically for healthy people who inherited severe, early-onset and often highly lethal illnesses that have a simple genetic component.
But immunity doesn't work that way, and, in the absence of vaccinations, inoculations, or genetic serendipity, populations "acquire resistance" to lethal illnesses the way the indigenous peoples of North and South America acquired resistance to smallpox.
Colorectal cancer is now in the top three potentially lethal illnesses in the rich world, and it is proved that vegetarians and people who eat less red and processed meat are slightly less likely to develop it.
Supporters of such measures have argued that children are especially vulnerable to being exposed to potentially lethal illnesses, and that contracting them in schools or kindergartens may also put their elderly relatives at serious risk.
For the first time, researchers have identified how important differences in a person's immune system – caused by a group of white blood cells called monocytes – can be used to predict who will recover well after surgery and who is likely to develop lethal illnesses.
