By a journey to Eritrea on 1 to 5 months recommend protection against the following infections:

Diphtheria
Diphtheria is a serious throat infection, which infects from person-to-person through the air. The vaccination should be less than 10 years old otherwise a booster is needed.
Read more about diphteria here.

Hepatitis A (infectious hepatitis)
Infectious hepatitis infects through contaminated food and water. Vaccination consists of two injections about 12 months apart, which protects for up to 25 years. The first vaccination protects for 12 months. 
Read more about hepatitis A here.
The hepatitis A vaccine can be combined with the vaccine against hepatitis B.

Hepatitis B
Hepatitis B infects through blood, sexual activities and, in small children, through saliva. The vaccination consists of 2 vaccinations about 4 weeks apart and a third 6 months later. The protection is for at least 25 years perhaps life long. 
Read more about hepatitis B here.
The hepatitis B vaccine can be combined with the vaccine against hepatitis A.

HIV in Africa
109 Danes were infected with HIV in Africa between 1991 and 99. Africa is the continent with most HIV infected people. Overall about 8% of the population is infected with infection rates reaching up to 80% in risk groups like prostitutes.
Read more about HIV here.

Meningitis
Meningitis due to meningococcus bacteria infect from person to person through the air. The vaccine protects against infection for 3 years after 1 vaccination. There are two vaccines: One protects only against type A and C. The other protects against A, C, W135 and Y and is used for travel to Saudi Arabia (especially Hajj) and West Africa.

Read more about meningitis here.

Tetanus
Tetanus is a complication to wounds contaminated by soil. If there has been a vaccination within the past 10 years it is not necessary to give a booster in case of wounds and accidents. Tetanus vaccinations are usually given in combination with diphtheria vaccine. If you are previously vaccinated, the vaccine can be given right up to departure.

Yellow fever - transit
Certain countries without yellow fever require a valid yellow fever vaccination certificate if you arrives (even in transit) from a country where yellow fever is present. 

If you arrive from a country without yellow fever, there is no requirement for a yellow fever vaccination.

Read more about yellow fever here.

Malaria in Eritrea
Risk of malaria in Erithrea is seen on the map below. The choice is between Malarone, Doxycycline or Lariam. Protection against mosquito bites will reduce the risk further.
The distribution of malaria can be seen at the map below.
Read more about malaria here.

The information on malaria risk in individual countries are based on the sources found here

Recommended vaccinations Starting Diphtheria 1 day Hepatitis A (infectious hepatitis) 1 week Hepatitis B 3 - 4 weeks HIV in Africa Meningitis 1 week Tetanus 1 day Yellow fever - transit 10 days Malaria in Eritrea 1-7 days 1-7 days When to start vaccinations before leaving::

Read more about the single vaccinations below.
If you need more information, Ask the Doctor