Blood parasites are malaria plasmodia, microfilaria species, trypanosomes (the causative agents of African sleeping sickness and South American Changas disease) and the causative agents of schistosomiasis of the bladder and the intestine. Their geographical distribution, incubation periods, signs and symptoms, microscopic and serological methods are described. In Germany around 1,000 tourists contract malaria every year, mostly travellers to Africa. Over 70% suffer from the life-threatening P. falciparum infection. Only a few days after the onset of this flu-like disease, complications may evolve. The best diagnostic method is the thin blood film. In case of a negative result this procedure must be repeated twice daily. The thick film requires experience. Rapid diagnostic tests can be helpful but are hampered by false negative results. Filaria loa loa may cause skin swellings, involvement of the eye and even the CNS; Wuchereria bancrofti can cause severe lymphedema. West African sleeping sickness (Trypanosoma gambiense) ends up in encephalitis, the East African form (T. rhodesiense) in a polyserositis. Schistosomiasis of the urinary bladder and the large intestine may cause severe diseases of the urinary tract or the liver.