Here's a list of the most common sexually transmitted diseases, together with the pathogens that cause them.

Name of STD	Causative agent or pathogen
urethritis	caused by a variety of pathogens
chlamydia infection	caused by special bacteria (Latin name: Chlamydia trachomatis)
fig warts (condyloma)	caused by a virus (Human Papilloma Virus, HPV)
genital herpes	caused by a herpes virus (Herpes Simplex Virus, HSV)
hepatitis (also known as inflammation of the liver)	caused by various hepatitis viruses (e.g. hepatitis A virus, HAV; hepatitis B virus, HBV, and hepatitis C virus, HCV)
AIDS / HIV	caused by the HI-virus (HIV standing for "human immunodeficiency virus")
syphilis (also known as lues)	caused by bacteria (Latin name: Treponema pallidum)
clap (also known as gonorrhoea)	caused by bacteria (gonococci or in Latin: Neisseria gonorrhoeae)

And what about Crabs & Co? Yes, you can pick these up during sex, too. The main ones are pubic lice (also known as crabs; Latin name: Phthirus pubis) and scabies (also called "The Itch"). In addition, there are trichonomad infections, which are also transmitted during sex, and fungal infections (mycosis and candida), which give rise to symptoms in genital areas.

Basically, once your health is undermined by one venereal disease, it's far easier for the pathogens of other STIs to attack you. So it's always a good idea to see the doctor straight away, even with minor symptoms.