The magnetic structure of the cubic helimagnet MnGe was studied by small-angle neutron scattering in a wide temperature range from 10 to 300 K. We show that MnGe undergoes a complex order-disorder phase transition covering more than 100 K above the critical temperature TN=(130±2) K. Moreover, the helical structure is intrinsically unstable below TN, since the profile of the reflection at kh∼2 nm-1 associated with the spin helix can be described by the sum of Gaussian and Lorentzian contributions, indicating a stable helix and spin-helix fluctuations, respectively. The Gaussian behavior determines the system at low temperature, whereas the Lorentzian is negligible. With increasing temperature, however, the Lorentzian starts to contribute and dominates at TN. The spin-helix fluctuations are accompanied by intensive spin excitations observed in small-angle neutron scattering as Q-independent scattering at Q<kh. The integral intensities of both the spin excitations and spin fluctuations obey exactly the same temperature law proportional to exp(T/Ta) with the activation temperature Ta=(80±1) K. These two contributions coexist above TN, being well distinguished up to TDM=150 K, where they are intermixed with and transform into diffuse scattering at TSR=(175±5) K. Above TSR the diffuse scattering is well described by a Gauss law, ascribed to static short-range ferromagnetic inhomogeneities with an inverse correlation length κ≈2 nm-1.