There were many negative impacts due to European colonization of Africa. Since the other user stated disease & slavery, I will only briefly talk about those (abuse is very much in the realms of slavery).
During the Colonial Era as well as the Age of Imperialism (which followed after), Europeans (specifically France, Spain, Britain, and later on Germany, Austrio-Hungary, etc.) were keen on exploring, creating colonies, and exploiting the resources offered by these 3rd world countries (of the time).
Africa, on one hand, took the brunt force of most of these colonies, as they suffered the longest colonization period. Towards the beginning of the colonization era, Africa was explored as Europeans had to go around the Cape Horn in an effort to find a oceanic route to China, where they hoped to open trade with. This led to European countries laying claim on African lands where they land, and building up colonies there. However, the Europeans soon had a falling apart with the tribes, which led to wars between the colonizers and the colonised. Usually, the Europeans were victorious (except for cases such as Ethiopia & Italy, but even then, they fail later on), and imprisoned many of the natives, leading to a large supply of slaves within the Middle East. As the states in the US soon expanded and grew into a country, many southern states soon had a growing market for these said states, which led to the enslavement of many fit Africans. (negative impact #1: enslavement of Africans).
On top of enslavement, the European countries soon introduced "cash crops" to the nations, which were crops (tobacco, cotton, and the like) that generated "cash", but not food, which the colonies dutifully grew, which delayed their economic growth, as they were not able to self-sustain, and their economy relied heavily on one or two crops (which can have a changing market demand; Remember the supply v. demand, in which prices vary depending on demand. If the demand is low, the price would generally be low). These crops were perfect for certain time periods, but soon fell out of favor, resulting in loss profits. These countries, with their only high-source of income being these cash-crops, soon lost profit, which resulted in a smaller economy than before.
Another negative impact that the Europeans had have in occupying the African Continent, was the amount of funding they gave to the nation. Excluding stronger economic nations in Africa (Egypt really), and the Middle East, most of the nations were not properly funded, and lacked development. This is extremely true, as most 3rd world countries are located in Africa and the Middle East. The way most power was given back to the people during the decolonization era also led to wars and coup d'tat across the continent, leading to an unstable economy and life for those who lived in it. Without proper decolonization methods, the countries soon fell into turmoil.
Finally, the African culture was heavily damaged due to European colonization. As most Europeans during that time were either Roman-Catholic or Protestant, the colonizers also brought their religions with them, and usually forced natives into believing in their same religion. While this is not a bad thing in itself, it led to a destruction of their native religion as well as their culture, and the way they lived.
All of these are different kinds of ways Europeans have a negative impact on their former African countries.
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