"sick" 和 "ill" 在许多情况下可以互换使用，因为它们都用来描述人体健康状态的不适或疾病。但是在某些语境下，它们可能有细微的区别。
一般来说：
1. "Sick" 通常更常用于口语，而 "ill" 则更常用于书面语。
2. "Sick" 有时可以指较短暂的不适或轻微的疾病，而 "ill" 更常用于指长期或严重的疾病。但在英式口语中，“sick” 也可以描述 “想吐的”，而 “ill” 则没有这层含义。
比如：
I’m very ill. I think I need to go to hospital.
我病得很严重，需要去医院。
My co-worker was sick, so I had to carry the ball.
我的搭档病了，所以我得负起全责。
In November 1941 Payne was seriously ill with pneumonia...
1941 年 11 月,佩恩得了严重的肺炎。
Her daughters visited him from time to time when he was ill.
他生病的时候她的女儿们时常去探望他。
She became a nun and cared for the terminally ill for the rest of her life.
她做了修女,用她的余生照顾患有绝症的人们。
I didn't shirk my duties. I visited the sick and I preached the gospel.
我没有逃避职责，我探望病人，也宣讲福音。《柯林斯高阶英汉双解学习词》
There were no doctors to treat the sick.
没有医生为病人医治。
fall ill, be taken ill 等动词词组中, 如
He fell ill shortly before Christmas .
他圣诞节前不久突然病倒了。
Shortly before Christmas, he was mysteriously taken ill.
圣诞节前不久,他突然不明原因地病倒了。
She was taken ill suddenly.
她突然病倒了。
One of the jury members was taken ill .
陪审团的一名成员突然病了。
She felt sick after eating too much candy.
她吃了太多糖果后感到不舒服。
He's been feeling sick all morning, but he refuses to go home.
他整个上午都感觉不舒服，但他拒绝回家。
She has been feeling ill for several days and finally decided to see a doctor.
她已经感觉不适好几天了，最终决定去看医生。
He was diagnosed with a serious illness last year and has been undergoing treatment since then.（他去年被诊断出患有严重的疾病，从那以后一直在接受治疗。）
be ill with 不利于
The company's conservative instincts sit ill with competition.
该公司保守的本性不利于竞争。
3. "ill" 通常只用来描述身体健康方面的问题，而"sick" 有时也可以用来形容对某种事物感到不舒服或厌恶的感觉，比如：
The thought of eating sushi makes me feel sick.一想到吃寿司我就感觉恶心。
I'm sick of listening to the same song over and over again.我厌倦了一遍又一遍地听同一首歌。
I'm sick and tired of your excuses.我对你的借口感到厌烦。
总的来说，这两个词大部分词义是重合的，但还是有细微区别。大家可以在日常阅读中多多去感受。
I feel ill. I need to sit down. 我感觉身体生病了，所以需要坐下来休息一下。
I feel sick. I need to sit down.我感觉恶心想吐，所以需要坐下来休息一下。
sick 常用在名词之前，如 sick children (生病的孩子)。在英国英语中, ill 比 sick 更礼貌委婉。sick 常表示生病时的身体感受, 例如恶心、呕吐等：
I spent the next 24 hours in bed, groaning and being sick .
接下来的 24 个小时我都躺在床上, 不停地呻吟，感到一阵阵恶心。
英国人常用 ill 的情况在美国英语中更常用 sick:
Some people get hurt in accidents or get sick .
一些人在事故中受伤或生病。
feel sick 感到难受、恶心
make someone sick 让某人不舒服、恶心
be sick 呕吐
I can't afford to get sick (= become ill) .
我病不起。
If you eat any more cake you'll make yourself sick .
你要是再吃蛋糕，就该吐了。
She got up and was sick in the handbasin.
她起身在洗手池里吐了起来。
Her mother's very sick.
她母亲病得很厉害。
Be sick of sb/sth | sick of doing sth（对…）厌倦的，厌烦的，厌恶的
I'm sick of the way you've treated me.
你对待我的那一套我都厌倦了。
I got sick of my boss's moaning and told him he could stick the job.
我烦透了老板的牢骚，便跟他说那活儿他自己干吧，我才不稀罕。
I am sick and tired of hearing all these people moaning.
我厌倦了听这些人大发牢骚。
We're sick of waiting around like this.
这么等来等去，我们感到很腻味。
I'm sick and tired of your moaning.
你的牢骚我都听腻了。
I'm sick of you prying into my personal life!
我讨厌你刺探我的私生活！
I'm sick to death of all of you!
你们全都烦死人了！
I'm sick to death of your endless criticism.
你这无休止的指责真让我烦死了。
make sb sick使厌恶；使反感；感到气愤
His hypocrisy makes me sick.
他的虚伪让我恶心。
The very thought of it makes me feel sick.
一想到这事就使我恶心。
It makes me sick that people commit offences and never get punished...
一些人屡屡犯罪却从未受到惩戒，这真让我气愤。
4.ill和sick都可以作定语或表语用。不过许多无知的出题者只知道sick能做定语，所以考生在遇到作定语的选择时还是选sick比较好，因为你选ill作定语判你个错误不得分，你和谁说理去呢？下面举一些它们都可以作定语的经典例句：
Man's life is full of the various good or ill luck.
人生充满着种种吉凶。《汉英大词典》
She had brought ill luck into her family.（有害的;讨厌的）
她给家庭带来了厄运。
The ill fated ship was sent off course into shallow waters and rammed by another vessel.
这艘倒霉的船偏离航道误入浅水区，被另外一艘船撞上了。
But it's an ill wind; I recovered and married one of my nurses from that hospital.
但是因祸得福：我恢复了健康，并娶了那所医院护理我的一位护士。
Many mentally ill people are themselves unhappy about the idea of community care
许多精神病患者本身不喜欢社区护理。
It is wrong that we have to tell ill people to go somewhere else to look for treatment
我们不得不让病人到别处求医的做法是不可取的。
The current arrangements for the care of severely mentally ill people are inadequate to meet their needs
目前对严重精神病患者的护理安排不足以满足他们的需要。
She kept vigil at the bedside of her critically ill son
她整夜都守在病重的儿子床边。
A couple with a terminally ill child want to create a designer baby that could save the boy's life.
孩子患了不治之症的一对夫妻想要培育一个设计婴儿来挽救儿子的性命。
以上9例摘自《柯林斯高阶英汉双解学习词典》
He has an ill temper.
他脾气很坏。《外研社英语同义词辨析词典》
I had appointments all morning, meeting with ill people. I had no time to think.
整个上午我都有预约，会见病人，我没有时间思考。
The sick woman moaned in pain.
那个生病的女人在痛苦地呻吟。
The mother is worried about her sick child.
母亲担心她那生病的孩子。
The woman's concern for her sick child is great.
那个妇女非常挂念生病的孩子。以上3例摘自《外研社英语同义词辨析词典》
Her sick child is a good anxiety to her.
她生病的孩子是她的一大心病。
I have a sick child on my hands.
我身边有个病孩要照料。
The sick man breathed his last at ten o'clock this morning.
这个病人是今天上午10点断气的。
The sick child moaned a little, and then fell asleep.
那个生病的孩子呻吟了一会儿就睡着了。
以上4例摘自《英汉大词典》
In a perfect world, there would be the facilities and money to treat every sick person.
理想的状况是有足够的设备和财力治疗每一位病人。
A sick person needs absolute confidence and trust in a doctor.
病人需要对医生有绝对的信心和信任。
The sick person gradually falls into a torpor.
病人逐渐变得萎靡。
Since Mr Bennett was going to need some time off work, he asked for a sick note
因为贝内特先生要请假一段时间，他请医生开了病假条。
She found herself with two small children, a sick husband, and no money
她发觉自己有两个年幼的孩子和一个有病在身的丈夫，却没钱养活他们。
以上5例《柯林斯高阶英汉双解学习词典》
I have a tame doctor who'll always give me a sick note when I want a day off.
我的医生对我有求必应。我想要休一天假时，他总会给我开病假条。《牛津高阶英汉双解词典》
As we move into the future, 3D printing will revolutionize the way we shop, the way we manufacture and the way we treat sick people.
随着我们迈向未来,3D打印将彻底改变我们的购物方式、生产方式和治疗病人的方式。
中考真题- 2019 贵州 阅读B
It can even help sick people get better.
它甚至可以帮助病人康复。
中考真题- 2018 江西 七选五
To do CPR, you press on the sick person's chest so that blood moves through the body and takes oxygen to organs ．
做心肺复苏术时,你要按压病人的胸部,使血液在其体内流动并将氧气带到各个器官。
高考真题- 2017 北京 阅读A
a sick joke
令人毛骨悚然的笑话
a sick mind
变态的心理
We live in a sick society.
我们生活在一个病态的社会里。《牛津高阶英汉双解词典》
I was again using the cold, unflinching eye of the camera to probe a sick society.
我再次通过照相机冷漠、坚定的镜头去探究这个病态的社会。《柯林斯高阶英汉双解学习词典》
Mark is not in the office today. He's on sick leave.
马克今天没来上班，他休病假了。
sick day：带薪病假；sick leave：病假；sick pay ：（雇主发放的）疾病津贴。
(as) sick as a dog病得很重；呕吐得厉害
And the situation will be even worse if they become sick as a dog.
如果他们病得很重,情况会更糟。
中考真题- 2015 包头 阅读C
He couldn't come to work yesterday because he was as sick as a dog.
他昨天不能来上班，因为他病得很严重。
Michael was sick as a dog after last night's barbecue.
昨天吃完烧烤之后，Michale吐得一塌糊涂。
The situation will be even worse if they become sick as a dog.
如果他们病得很重，情况会更糟。
(as) sick as a parrot大失所望；失望透顶（parrot鹦鹉）
If you say that you feel as sick as a parrot, you feel very disappointed about something.
Mary is as sick as a parrot about losing her watch.
玛丽丢了表之后感到非常不高兴。
When I first came to the college, I was sick as a parrot.
当我第一次来到大学后，我感到非常失望。
John is really jealous of my new house-he's as sick as a parrot.
约翰非常嫉妒我有了一栋新房子，他是个十足的妒忌鬼。
He was sick as a parrot when his son failed to pass the final exam.
他儿子期末考试不及格后，他非常失望。
掌握以上知识点以后遇到ill和sick的考题都可以秒杀了。