serious illness

You use it as a noun phrase in a sentence. You can say someone 'has' a serious illness. You can also 'suffer from' or 'recover from' one. It works perfectly in both writing and speaking. It is a very direct way to talk about health. Just remember to use it with empathy. Nobody likes hearing bad news bluntly.

Use it when you need to explain why someone is away. It is great for HR meetings or doctor visits. Use it when talking about insurance or medical history. It is also common in news reports about public figures. If a friend is very sick, you might use it to tell others. It helps people understand they should be supportive and quiet.

Do not use it for a common cold. If you have a sniffle, do not say you have a serious illness. Your friends will think you are being a 'drama queen'. Avoid it in very light, happy conversations unless necessary. It can be a real 'mood killer' at a party. Also, do not use it if you want to be specific. Say 'pneumonia' instead if you know the name.

In English-speaking cultures, health is often a private matter. Using the phrase serious illness is a way to be honest without oversharing. It gives you privacy. You tell people it is bad without giving gross details. It is a polite shield. People will usually stop asking questions once they hear this phrase. They know it means 'respect my space'.

You might hear severe illness or critical condition. Chronic illness is used for long-term problems. In casual talk, people might say someone is 'really sick'. In medical settings, you will see acute illness. All of these describe different 'flavors' of being unwell. But serious illness remains the most common, all-purpose term for big health scares.