A 5-year-old boy vomiting blood has become the first cross-border case of Ebola in the current deadly outbreak. Now authorities in Congo are trying to determine how his family, exposed to the virus, managed to cross into neighboring Uganda.

The World Health Organization late Tuesday confirmed the first Ebola case outside Congo since the outbreak began in August. Nearly 1,400 people have died.

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This photo taken Thursday, April 18, 2019 and released by the International Rescue Committee (IRC), shows the shores of Lake Albert near the border with Congo, at Kabakanga in Kagadi district, western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

A 5-year-old boy vomiting blood has become the first cross-border case of Ebola in the current deadly outbreak. Now authorities in Congo are trying to determine how his family, exposed to the virus, managed to cross into neighboring Uganda.

This photo taken Thursday, April 18, 2019 and released by the International Rescue Committee (IRC), shows an Ebola-prevention information sign at the Ndaiga Health Centre II, near the shores of Lake Albert and the border with Congo, in Kagadi district, western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

But six managed to leave while awaiting transfer to an Ebola treatment center. Authorities say they entered Uganda, where the boy is receiving treatment and relatives are isolated.

This photo taken Monday, June 10, 2019 and released by the International Rescue Committee (IRC), shows an Ebola screening checkpoint where people crossing from Congo go through foot and hand washing with a chlorine solution and have their temperature taken, at the Bunagana border crossing with Congo, in western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

This photo taken Monday, June 10, 2019 and released by the International Rescue Committee (IRC), shows an Ebola screening checkpoint where people crossing from Congo go through foot and hand washing with a chlorine solution and have their temperature taken, at the Bunagana border crossing with Congo, in western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

This photo taken Tuesday, June 11, 2019 and released by the International Rescue Committee (IRC), shows the Matanda Transit Center for newly arrived asylum seekers where arrivals, like all people crossing from Congo, go through foot and hand washing with a chlorine solution and have their temperature taken, in western Uganda near the border with Congo. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

This photo taken Tuesday, June 11, 2019 and released by the International Rescue Committee (IRC), shows the Matanda Transit Center for newly arrived asylum seekers where arrivals, like all people crossing from Congo, go through foot and hand washing with a chlorine solution and have their temperature taken, in western Uganda near the border with Congo. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

In this photo taken Tuesday, June 11, 2019 and released by the International Rescue Committee (IRC), a sign is attached to a window of a new Ebola treatment unit currently under construction at the Kihihi Health Centre IV in Kanungu district, western Uganda, near the border with Congo. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

In this photo taken Tuesday, June 11, 2019 and released by the International Rescue Committee (IRC), a sign is attached to a window of a new Ebola treatment unit currently under construction at the Kihihi Health Centre IV in Kanungu district, western Uganda, near the border with Congo. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

Congo's health ministry says a dozen members of the boy's family had showed symptoms of Ebola and were put in isolation.

This photo taken Thursday, April 18, 2019 and released by the International Rescue Committee (IRC), shows the shores of Lake Albert near the border with Congo, at Kabakanga in Kagadi district, western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

This photo taken Thursday, April 18, 2019 and released by the International Rescue Committee (IRC), shows the shores of Lake Albert near the border with Congo, at Kabakanga in Kagadi district, western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

But six managed to leave while awaiting transfer to an Ebola treatment center. Authorities say they entered Uganda, where the boy is receiving treatment and relatives are isolated.

Experts have long feared Ebola could spread to neighboring countries because of unrest hampering response work in Congo.

This photo taken Thursday, April 18, 2019 and released by the International Rescue Committee (IRC), shows an Ebola-prevention information sign at the Ndaiga Health Centre II, near the shores of Lake Albert and the border with Congo, in Kagadi district, western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

This photo taken Thursday, April 18, 2019 and released by the International Rescue Committee (IRC), shows an Ebola-prevention information sign at the Ndaiga Health Centre II, near the shores of Lake Albert and the border with Congo, in Kagadi district, western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

This photo taken Monday, June 10, 2019 and released by the International Rescue Committee (IRC), shows an Ebola screening checkpoint where people crossing from Congo go through foot and hand washing with a chlorine solution and have their temperature taken, at the Bunagana border crossing with Congo, in western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

This photo taken Monday, June 10, 2019 and released by the International Rescue Committee (IRC), shows an Ebola screening checkpoint where people crossing from Congo go through foot and hand washing with a chlorine solution and have their temperature taken, at the Bunagana border crossing with Congo, in western Uganda. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

This photo taken Tuesday, June 11, 2019 and released by the International Rescue Committee (IRC), shows the Matanda Transit Center for newly arrived asylum seekers where arrivals, like all people crossing from Congo, go through foot and hand washing with a chlorine solution and have their temperature taken, in western Uganda near the border with Congo. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

This photo taken Tuesday, June 11, 2019 and released by the International Rescue Committee (IRC), shows the Matanda Transit Center for newly arrived asylum seekers where arrivals, like all people crossing from Congo, go through foot and hand washing with a chlorine solution and have their temperature taken, in western Uganda near the border with Congo. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

In this photo taken Tuesday, June 11, 2019 and released by the International Rescue Committee (IRC), a sign is attached to a window of a new Ebola treatment unit currently under construction at the Kihihi Health Centre IV in Kanungu district, western Uganda, near the border with Congo. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)

In this photo taken Tuesday, June 11, 2019 and released by the International Rescue Committee (IRC), a sign is attached to a window of a new Ebola treatment unit currently under construction at the Kihihi Health Centre IV in Kanungu district, western Uganda, near the border with Congo. Uganda's health ministry said late Tuesday, June 11, 2019 that a child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year. (Ben WiseInternational Rescue Committee via AP)