Obese man's heart, computer illustration.

Embryo during the late period of the 4th week, computer illustration.

Embryo during the late period of the 4th week, computer illustration.

Embryo during the late period of the 4th week, computer illustration.

Embryo during the late period of the 4th week, computer illustration.

Embryo during the middle part of the 4th week, computer illustration.

Embryo during the middle part of the 4th week, computer illustration.

Embryo during the middle part of the 4th week, computer illustration.

Embryo during the middle part of the 4th week, computer illustration.

Embryo during the middle part of the 4th week, computer illustration.

Human immunodeficiency viruses (HIV) infecting human embryo, conceptual illustration. The embryo is 4 weeks old.

Human immunodeficiency viruses (HIV) infecting human embryo, conceptual illustration. The embryo is 4 weeks old.

Embryo during the middle part of the 4th week, computer illustration.

Embryo during the middle part of the 4th week, computer illustration.

Viruses infecting human embryo, conceptual illustration. The embryo is 4 weeks old. Many viruses cause fetal abnormalities or stillbirth. For example, rubella, herpes and cytomegalovirus cause different fetal pathology, whilst fetal measles infection usually leads to stillbirth.

Viruses infecting human embryo, conceptual illustration. The embryo is 4 weeks old. Many viruses cause fetal abnormalities or stillbirth. For example, rubella, herpes and cytomegalovirus cause different fetal pathology, whilst fetal measles infection usually leads to stillbirth.

Viruses infecting human embryo, conceptual illustration. The embryo is 4 weeks old. Many viruses cause fetal abnormalities or stillbirth. For example, rubella, herpes and cytomegalovirus cause different fetal pathology, whilst fetal measles infection usually leads to stillbirth.

Computer illustration of a human eyeball.

Computer illustration of a human eyeball.

Gonorrhoea infection. Computer illustration showing discharge from a penis caused by the sexually transmitted disease gonorrhoea. Gonorrhoea is caused by the bacteria Neisseria gonorrhoeae. In many cases it is symptomless, but may cause painful urination or a purulent discharge, as shown here. In severe cases it may also cause inflammation of the testicles and prostate gland, and infertility. Treatment is with antibiotics.