Undoubtedly the jinn can have a harmful effect on humans

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:

“Undoubtedly the jinn can have a harmful effect on humans, and they could even kill them. They may harm a person by throwing stones at him, or by trying to terrify him, and other things that are proven in the Sunnah or indicated by real events. It was reported that the Messenger (peace and blessings of Allaah be upon him) gave permission to one of his Companions to go to his wife during one of the military campaigns — I think it was the campaign of al-Khandaq — as he was a young man who had recently got married. When he reached his house, he found his wife standing at the door, and he objected to that. She said to him, ‘Go inside,’ so he went inside and found a snake curled up on the bed. He had a spear with him, so he stabbed it with the spear until it died, and at the same instant as the snake died the man also died. It was not known which of them died first, the snake or the man. When the Prophet (peace and blessings of Allaah be upon him) heard of that, he forbade killing the harmless kinds of snakes that are found in houses, apart from those which are maimed or are streaked and malignant.

This indicates that the jinn may attack humans, and that they may harm them, as is known from real-life events. There are numerous reports which indicate that a man may come to a deserted area, and a stone may be thrown at him, but he does not see anybody, or he may hear voices or a rustling sound like the rustling of trees, and other things that may make him feel distressed and scared. A jinn may also enter the body of a human, either because of love or with the intention of harming him, or for some other reason. This is indicated in the aayah (interpretation of the meaning):

“Those who eat Ribaa will not stand (on the Day of Resurrection) except like the standing of a person beaten by Shaytaan (Satan) leading him to insanity”[al-Baqarah 2:275]

In such cases, the jinn may speak from inside of that person and address the one who is reading verses from the Qur’aan over him; the reciter may take a promise from the jinn never to come back, and other things concerning which there are very many reports which are widespread among the people.

So the protection which will prevent the evil of the jinn is for a person to recite that which is narrated in the Sunnah as being effective in providing protection, such as Aayat al-Kurisy, for if a person recites Aayat al-Kursiy at night, he will continue to have protection from Allaah, and no shaytaan will come near him until morning. And Allaah is the Protector.'”

(Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 1/287-288).